misc: Add explicit overrides and fix other clang >= 3.5 issues

This patch adds explicit overrides as this is now required when using
"-Wall" with clang >= 3.5, the latter now part of the most recent
XCode. The patch consequently removes "virtual" for those methods
where "override" is added. The latter should be enough of an
indication.

As part of this patch, a few minor issues that clang >= 3.5 complains
about are also resolved (unused methods and variables).

misc: Remove redundant compiler-specific defines

This patch moves away from using M5_ATTR_OVERRIDE and the m5::hashmap
(and similar) abstractions, as these are no longer needed with gcc 4.7
and clang 3.1 as minimum compiler versions.

sim: Refactor the serialization base class

Objects that are can be serialized are supposed to inherit from the
Serializable class. This class is meant to provide a unified API for
such objects. However, so far it has mainly been used by SimObjects
due to some fundamental design limitations. This changeset redesigns
to the serialization interface to make it more generic and hide the
underlying checkpoint storage. Specifically:

* Add a set of APIs to serialize into a subsection of the current
object. Previously, objects that needed this functionality would
use ad-hoc solutions using nameOut() and section name
generation. In the new world, an object that implements the
interface has the methods serializeSection() and
unserializeSection() that serialize into a named /subsection/ of
the current object. Calling serialize() serializes an object into
the current section.

* Move the name() method from Serializable to SimObject as it is no
longer needed for serialization. The fully qualified section name
is generated by the main serialization code on the fly as objects
serialize sub-objects.

* Add a scoped ScopedCheckpointSection helper class. Some objects
need to serialize data structures, that are not deriving from
Serializable, into subsections. Previously, this was done using
nameOut() and manual section name generation. To simplify this,
this changeset introduces a ScopedCheckpointSection() helper
class. When this class is instantiated, it adds a new /subsection/
and subsequent serialization calls during the lifetime of this
helper class happen inside this section (or a subsection in case
of nested sections).

* The serialize() call is now const which prevents accidental state
manipulation during serialization. Objects that rely on modifying
state can use the serializeOld() call instead. The default
implementation simply calls serialize(). Note: The old-style calls
need to be explicitly called using the
serializeOld()/serializeSectionOld() style APIs. These are used by
default when serializing SimObjects.

* Both the input and output checkpoints now use their own named
types. This hides underlying checkpoint implementation from
objects that need checkpointing and makes it easier to change the
underlying checkpoint storage code.

energy: Tighter checking of levels for DFS systems

There are cases where users might by accident / intention specify less voltage
operating points thatn frequency points. We consider one of these cases
special: giving only a single voltage to a voltage domain effectively renders
it as a static domain. This patch adds additional logic in the auxiliary parts
of the functionality to handle these cases properly (simple driver asking for
N>1 operating levels, we should return the same voltage for all of them) and
adds error checking code in the voltage domain.

energy: Small extentions and fixes for DVFS handler

These additions allow easier interoperability with and querying from an
additional controller which will be in a separate patch. Also adding warnings
for changing the enabled state of the handler across checkpoint / resume and
deviating from the state in the configuration.

Contributed-by: Akash Bagdia <akash.bagdia@arm.com>

power: Add basic DVFS support for gem5

Adds DVFS capabilities to gem5, by allowing users to specify lists for
frequencies and voltages in SrcClockDomains and VoltageDomains respectively.
A separate component, DVFSHandler, provides a small interface to change
operating points of the associated domains.

Clock domains will be linked to voltage domains and thus allow separate clock,
but shared voltage lines.

Currently all the valid performance-level updates are performed with a fixed
transition latency as specified for the domain.

Config file example:
...
vd = VoltageDomain(voltage = ['1V','0.95V','0.90V','0.85V'])
tsys.cluster1.clk_domain.clock = ['1GHz','700MHz','400MHz','230MHz']
tsys.cluster2.clk_domain.clock = ['1GHz','700MHz','400MHz','230MHz']
tsys.cluster1.clk_domain.domain_id = 0
tsys.cluster2.clk_domain.domain_id = 1
tsys.cluster1.clk_domain.voltage_domain = vd
tsys.cluster2.clk_domain.voltage_domain = vd
tsys.dvfs_handler.domains = [tsys.cluster1.clk_domain,
tsys.cluster2.clk_domain]
tsys.dvfs_handler.enable = True

sim: Expose the current voltage for each object as a stat

power: Add voltage domains to the clock domains

This patch adds the notion of voltage domains, and groups clock
domains that operate under the same voltage (i.e. power supply) into
domains. Each clock domain is required to be associated with a voltage
domain, and the latter requires the voltage to be explicitly set.

A voltage domain is an independently controllable voltage supply being
provided to section of the design. Thus, if you wish to perform
dynamic voltage scaling on a CPU, its clock domain should be
associated with a separate voltage domain.

The current implementation of the voltage domain does not take into
consideration cases where there are derived voltage domains running at
ratio of native voltage domains, as with the case where there can be
on-chip buck/boost (charge pumps) voltage regulation logic.

The regression and configuration scripts are updated with a generic
voltage domain for the system, and one for the CPUs.