arm/arm64: GIC: basic GICv2 MMIO tests

This adds an MMIO subtest to the GIC test.
It accesses some generic GICv2 registers and does some sanity tests,
like checking for some of them being read-only.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>