[ARM] vgic: First-cut implementation of host-to-guest irq routing

This patch introduces host-to-guest irq routing for VGIC. This
implementation does not break existing functionality of VGIC.

We still need to do more testing for host-to-guest irq routing
implemented by this patch hence it is still work-in-progress.

Signed-off-by: Anup Patel <anup@brainfault.org>