feat(smmu): configure SMMU Root interface

This change performs a basic configuration of the SMMU root registers
interface on an RME enabled system. This permits enabling GPC checks
for transactions originated from a non-secure or secure device upstream
to an SMMU. It re-uses the boot time GPT base address and configuration
programmed on the PE.
The root register file offset is platform dependent and has to be
supplied on a model command line.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I4f889be6b7afc2afb4d1d147c5c1c3ea68f32e07