KVM: selftests: aarch64: Add function for accessing GICv3 dist and redist registers

Add a generic library function for reading and writing GICv3 distributor
and redistributor registers. Then adapt some functions to use it; more
will come and use it in the next commit.

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/20211109023906.1091208-3-ricarkol@google.com