arm: prepare for instantiating different IRQ chip devices

Extend the vGIC handling code to potentially deal with different IRQ
chip devices instead of hard-coding the GICv2 in.
We extend most vGIC functions to take a type parameter, but still put
GICv2 in at the top for the time being.

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