The only difference between GIC-500 and GIC-600 relevant to TF-A is the
differing power management sequence.
A certain GIC implementation is detectable at runtime, for instance by
checking the IIDR register. Let's add that test before initiating the
GIC-600 specific sequence, so the code can be used on both GIC-600 and
GIC-500 chips alike, without deciding on a GIC chip at compile time.

This means that the GIC-500 "driver" is now redundant. To allow minimal
platform support, add a switch to disable GIC-600 support.

Change-Id: I17ea97d9fb05874772ebaa13e6678b4ba3415557
Signed-off-by: Andre Przywara <andre.przywara@arm.com>