refactor(gic): move GIC IIDR numbers

For the GIC power management we need to identify certain GIC
implementations, so we have the IIDR values for some Arm Ltd. GIC models
defined.
We will need those number elsewhere very soon, so export them to a
shared header file, to avoid defining them again.

Change-Id: I1b8e2d93d6cea0d066866143c89eef736231134f
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
2 files changed