KVM: arm64: vgic-its: KVM_DEV_ARM_VGIC_GRP_ITS_REGS group

The ITS KVM device exposes a new KVM_DEV_ARM_VGIC_GRP_ITS_REGS
group which allows the userspace to save/restore ITS registers.

At this stage the get/set/has operations are not yet implemented.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Christoffer Dall <cdall@linaro.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>