ARM: vITS: create ITS subnodes for Dom0 DT

ARM: vITS: create ITS subnodes for Dom0 DT

Dom0 expects all ITSes in the system to be propagated to be able to
use MSIs.
Create Dom0 DT nodes for each hardware ITS, keeping the register frame
address the same, as the doorbell address that the Dom0 drivers program
into the BARs has to match the hardware.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>