KVM: arm: vgic-v3: Add support for ICC_SGI0R and ICC_ASGI1R accesses

In order to generate Group0 SGIs, let's add some decoding logic to
access_gic_sgi(), and pass the generating group accordingly.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>