gpio: add interrupt handling capability to max732x

Most of the GPIO expanders supported by the max732x driver have interrupt
generation capability by reporting changes on input pins through an INT#
pin.  This patch implements the irq_chip functionnality (edge detection
only).

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Jebediah Huang <jebediah.huang@gmail.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>