ARM: dts: imx6sx-sdb: enable gpio buttons as wakeup source

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>

arch/arm/boot/dts/imx6sx-sdb.dtsi

@@ -40,12 +40,14 @@ volume-up {
 			label = "Volume Up";
 			gpios = <&gpio1 18 GPIO_ACTIVE_LOW>;
 			linux,code = <KEY_VOLUMEUP>;
+			wakeup-source;
 		};
 
 		volume-down {
 			label = "Volume Down";
 			gpios = <&gpio1 19 GPIO_ACTIVE_LOW>;
 			linux,code = <KEY_VOLUMEDOWN>;
+			wakeup-source;
 		};
 	};