add support for supplying GICv3 redistributor addresses

The code currently is assuming fixed sized memory regions for the
distributor and CPU interface. GICv3 needs a dynamic allocation of
it's redistributor region, since it's size depends on the number of
vCPUs.
Also add the necessary code to create a GICv3 IRQ chip instance.

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