armv8: SPL: only compile GIC code if needed

Not every SoC needs to set up the GIC interrupt controller, so link
think code only when the respective config option is set.
This shaves off some bytes from the SPL code size.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>