dev, arm: Add a customizable NoMali GPU model

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>