Add function definition for irq_chip.set_wake to enable GPIO to
wake-up the system.

This patch has been tested on a MX51 Babbage system that had suspend
code implemented. The set_wake implementation is necessary for a
GPIO to wake up a system from suspend.
Signed-off-by: Dinh Nguyen <Dinh.Nguyen@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>