arm: gic: allow 32-bit compilation

The ARM GIC code is shared between ARM and arm64, though the header
files from the Linux kernel are actually different, reflecting the
different level of GIC emulation support between the two architectures.
Since it is conceivable that ARM will gain GICv3 (and possibly ITS)
emulation support too, lets conditionally define those missing
symbols in the source file until an upcoming ARM header update eventually
includes those.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>