arm64: dts: fvp: Fix GIC child nodes

The GIC DT nodes for the fastmodels were not fully compliant with the
DT binding, which has certain expectations about child nodes and their
size and address cells values.

Use smaller #address-cells and #size-cells values, as the binding
requests, and adjust the reg properties accordingly.
This requires adjusting the interrupt nexus nodes as well, as one
field of the interrupt-map property depends on the GIC's address-size.

Since the .dts files share interrupt nexus nodes across different
interrupt controllers (GICv2 vs. GICv3), we need to use the only
commonly allowed #address-size value of <1> for both.

Link: https://lore.kernel.org/r/20200513103016.130417-11-andre.przywara@arm.com


Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>