arm/arm64: GICv2: add GICD_IPRIORITYR testing

Some tests for the IPRIORITY registers. The significant number of bits
is IMPLEMENTATION DEFINED, but should be the same for every IRQ.
Also these registers must be byte-accessible.
Check that accesses beyond the implemented IRQ limit are actually
read-as-zero/write-ignore.

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