arm64: dts: prepare foundation-v8.dts to cope with GICv3

To prepare the ARM foundation model to support GICv3, we adjust
the #address-cells property of the current GICv2 node to be
compatible with the two cells required for GICv3 later.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>