rpi3: Prepare for supporting a GIC (in RPi4)

As the PSCI "power" management functions for the Raspberry Pi 3 port
will be shared with the upcoming RPi4 support, we need to prepare them
for dealing with the GIC interrupt controller.
Splitting this code just for those simple calls to the generic GIC
routines does not seem worthwhile, so just use a #define the protect the
GIC code from being included by the existing RPi3 code.

Change-Id: Iaca6b0214563852b28ad4a088ec45348ae8be40d
Signed-off-by: Andre Przywara <andre.przywara@arm.com>