Instead of hardcoding the shifts and masks in the GICD_IIDR register
emulation, let's add the definition of these fields to the GIC header
files and use them.

This will make things more obvious when we're going to bump the revision
in the IIDR when we'll make guest-visible changes to the implementation.
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>