KVM: arm64: vgic-its: Implement vgic_mmio_uaccess_write_its_creadr

GITS_CREADR needs to be restored so let's implement the associated
uaccess_write_its callback. The write only is allowed if the its
is disabled.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Christoffer Dall <cdall@linaro.org>