add snvs power key driver.
It work in imx chips after i.mx6sx

ON/OFF key used power on/off whole system.
This driver make it wakeup from suspend state when short press
ON/OFF key.

Long time press will trig SNVS power off chip without software
intervention.
Signed-off-by: Robin Gong <b38343@freescale.com>
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>