irqchip/gic-v3-its: Add probing for VLPI properties

Add the probing code for the ITS VLPI support. This includes
configuring the ITS number if not supporting the single VMOVP
command feature.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>