ARM: GIC: export and extend vgic_init_pending_irq()

ARM: GIC: export and extend vgic_init_pending_irq()

For LPIs we later want to dynamically allocate struct pending_irqs.
So beside needing to initialize the struct from there we also need
to clean it up and re-initialize it later on.
Export vgic_init_pending_irq() and extend it to be reusable.

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