arm64: KVM: vgic: add GICv3 world switch

Introduce the GICv3 world switch code used to save/restore the
GICv3 context.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>