irqchip/gic-v3-its: Add VPE domain infrastructure

Add the basic GICv4 VPE (vcpu in GICv4 parlance) infrastructure
(irqchip, irq domain) that is going to be populated in the following
patches.

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