hw/intc: add implementation of GICD_IIDR to Arm GIC

hw/intc: add implementation of GICD_IIDR to Arm GIC

a66a24585f (hw/intc/arm_gic: Implement read of GICC_IIDR) implemented
this for the CPU interface register. The fact we don't implement it
shows up when running Xen with -d guest_error which is definitely
wrong because the guest is perfectly entitled to read it.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>