arm/arm64: add initial gicv2 support

Add some gicv2 support. This just adds init and enable
functions, allowing unit tests to start messing with it.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>