arm: FDT: create MSI controller DT node

The ARM GICv3 ITS requires a separate device tree node to describe
the ITS. Add this as a child to the GIC interrupt controller node
to let a guest discover and use the ITS if the user requests it.
Since we now need to specify #address-cells for the GIC node, we
have to add two zeroes to the interrupt map to match that.

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