irqchip/gic: Add .irq_ack() to GIC-based irqchips

Subsequent patches will make the GIC IRQs use a flow handler that issues an
irq_ack(), thus irqchips of child domains need to have an .irq_ack() of
their own.

Most cases are covered by the default MSI callbacks, but nexus domains and
other non-MSI domains still need tending do.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>