KVM: ARM: vgic: add the GICv3 backend

Introduce the support code for emulating a GICv2 on top of GICv3
hardware.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

diff --cc arch/arm64/include/asm/kvm_asm.h
Simple merge

diff --cc arch/arm64/kvm/vgic-v3-switch.S
Simple merge

diff --cc include/kvm/arm_vgic.h
Simple merge

diff --cc virt/kvm/arm/vgic-v3.c
Simple merge