arm64: KVM: vgic: byteswap GICv2 access on world switch if BE

Ensure that accesses to the GICH_* registers are byteswapped
when the kernel is compiled as big-endian.

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