KVM: selftests: aarch64: Add GICv3 register accessor library functions

Add library functions for accessing GICv3 registers: DIR, PMR, CTLR,
ISACTIVER, ISPENDR.

Signed-off-by: Ricardo Koller <ricarkol@google.com>
Acked-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211109023906.1091208-4-ricarkol@google.com