dev: Build the PCI device models even in NULL_ISA builds.

There are some minor ISA dependencies in the PCI device models,
specifically that they use the set<> accessors on the packet objects.
This actually compiles fine because the NULL ISA claims to be little
endian, but really these accessors should be changed to use little
endian all the time since that's what PCI is defined to use, not
the guest endianness.

The other types of accessors, specifically the ones that default to
what the guest wants, should be excluded when building NULL_ISA, and,
pending other dependencies, the NULL_ISA should no longer have an
endianness associated with it.

Change-Id: I0739122dbf67d109e7959553a1eff0239b090ca4
Reviewed-on: https://gem5-review.googlesource.com/c/13468
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

dev: Move the CopyEngine class to src/dev/pci

dev: Move existing PCI device functionality to src/dev/pci

Move pcidev.(hh|cc) to src/dev/pci/device.(hh|cc) and update existing
devices to use the new header location. This also renames the PCIDEV
debug flag to have a capitalization that is consistent with the PCI
host and other devices.

dev: Rewrite PCI host functionality

The gem5's current PCI host functionality is very ad hoc. The current
implementations require PCI devices to be hooked up to the
configuration space via a separate configuration port. Devices query
the platform to get their config-space address range. Un-mapped parts
of the config space are intercepted using the XBar's default port
mechanism and a magic catch-all device (PciConfigAll).

This changeset redesigns the PCI host functionality to improve code
reuse and make config-space and interrupt mapping more
transparent. Existing platform code has been updated to use the new
PCI host and configured to stay backwards compatible (i.e., no
guest-side visible changes). The current implementation does not
expose any new functionality, but it can easily be extended with
features such as automatic interrupt mapping.

PCI devices now register themselves with a PCI host controller. The
host controller interface is defined in the abstract base class
PciHost. Registration is done by PciHost::registerDevice() which takes
the device, its bus position (bus/dev/func tuple), and its interrupt
pin (INTA-INTC) as a parameter. The registration interface returns a
PciHost::DeviceInterface that the PCI device can use to query memory
mappings and signal interrupts.

The host device manages the entire PCI configuration space. Accesses
to devices decoded into the devices bus position and then forwarded to
the correct device.

Basic PCI host functionality is implemented in the GenericPciHost base
class. Most platforms can use this class as a basic PCI controller. It
provides the following functionality:

* Configurable configuration space decoding. The number of bits
dedicated to a device is a prameter, making it possible to support
both CAM, ECAM, and legacy mappings.

* Basic interrupt mapping using the interruptLine value from a
device's configuration space. This behavior is the same as in the
old implementation. More advanced controllers can override the
interrupt mapping method to dynamically assign host interrupts to
PCI devices.

* Simple (base + addr) remapping from the PCI bus's address space to
physical addresses for PIO, memory, and DMA.