arm64: dts: juno: Fix GIC child nodes

arm64: dts: juno: Fix GIC child nodes

The GIC DT nodes for the Juno boards 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.

Link: https://lore.kernel.org/r/20200513103016.130417-10-andre.przywara@arm.com
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>