This patch enables i.MX6SX SDB board's below GPIO buttons
as wakeup sources:

SW4(FUNC1): KEY_VOLUMEUP
SW5(FUNC2): KEY_VOLUMEDOWN
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>