KVM: arm/arm64: vgic: Keep track of implementation revision

As we are about to tweak implementation aspects of the VGIC emulation,
while still preserving some level of backwards compatibility support,
add a field to keep track of the implementation revision field which is
reported to the VM and to userspace.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>