arm64: dts: fvp: Fix ITS node names and #msi-cells

The GIC ITS nodes in the fastmodel DTS files were not fully binding
compliant.

Use one of the allowed node names, also add the required #msi-cells
property for the older model.

Link: https://lore.kernel.org/r/20200513103016.130417-12-andre.przywara@arm.com


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