KVM: arm-vgic: Support CPU interface reg access

KVM: arm-vgic: Support CPU interface reg access

Implement support for the CPU interface register access driven by MMIO
address offsets from the CPU interface base address.  Useful for user
space to support save/restore of the VGIC state.

This commit adds support only for the same logic as the current VGIC
support, and no more.  For example, the active priority registers are
handled as RAZ/WI, just like setting priorities on the emulated
distributor.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
(cherry picked from commit fa20f5aea56f271f83e91b9cde00f043a5a14990)
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>