[ARM] Orion: top-level IRQs are level-triggered

Make it clear that Orion top-level IRQs are level-triggered.  This
means that we don't need an ->ack() handler, or at least, we don't
need the ->ack() handler (or the acking part of the ->mask_ack()
handler) to actually do anything.

Given that, we might as well point our ->mask_ack() handler at the
->mask() handler instead of providing a dummy ->ack() handler, since
providing a ->mask_ack() handler on level IRQ sources will prevent
->ack() from ever being called.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Russell King <linux@arm.linux.org.uk>