dev, arm: Add a customizable NoMali GPU model

Add a customizable NoMali GPU model and an example Mali T760
configuration. Unlike the normal NoMali model (NoMaliGpu), the
NoMaliCustopmGpu model exposes all the important GPU ID registers to
Python. This makes it possible to implement custom GPU configurations
by without changing the underlying NoMali library.

Change-Id: I4fdba05844c3589893aa1a4c11dc376ec33d4e9e
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Andreas Hansson <andreas.hansson@arm.com>

dev, arm: Implement the NoMali reset callback

Add a callback handler for the NoMali reset callback. This callback is
called whenever the GPU is reset using the register interface or the
NoMali API. The callback can be used to override ID registers using
the raw register API.

dev, arm: Refactor the NoMali GPU

Refactor and cleanup the NoMaliGpu class:

* Use a std::map instead of a switch block to map the parameter enum
describing the GPU type to a NoMali type.

* Remove redundant NoMali handle from the interrupt callback.

* Make callbacks and API wrappers protected instead of private to
enable future extensions.

* Wrap remaining NoMali API calls.

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.

dev, arm: Add a device model that uses the NoMali model

Add a simple device shim that interfaces with the NoMali model
library. The gem5 side of the interface supports Mali T60x/T62x/T760
GPUs. This device model pretends to be a Mali GPU, but doesn't render
anything and executes in zero time.