KVM: arm/arm64: vgic-new: Add GICv3 IDREGS register handler

We implement the only one ID register that is required by the
architecture, also this is the one that Linux actually checks.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>