arm64: enable GICv3-ITS emulation

With everything in place for the ITS emulation add a new option to the
--irqchip parameter to allow the user to specify --irqchip=gicv3-its
to enable the ITS emulation.
This will trigger creating the FDT node and an ITS register frame to
tell the kernel we want ITS emulation in the guest.

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