python: Don't assume SimObjects live in the global namespace

The importer in Python 3 doesn't like the way we import SimObjects
from the global namespace. Convert the existing SimObject declarations
to import from m5.objects. As a side-effect, this makes these files
consistent with configuration files.

Change-Id: I11153502b430822130722839e1fa767b82a027aa
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15981
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

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: 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.