ARM: dts: imx6sx: correct property name for wakeup source

Commit(def56bba input: snvs_pwrkey: use "wakeup-source"
as deivce tree property name) replaces the property name
of "wakeup" with "wakeup-source", update this change
in i.MX6SX dtsi accordingly.
Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6sx.dtsi

@@ -686,7 +686,7 @@ snvs_pwrkey: snvs-powerkey {
 					regmap = <&snvs>;
 					interrupts = <GIC_SPI 4 IRQ_TYPE_LEVEL_HIGH>;
 					linux,keycode = <KEY_POWER>;
-					wakeup;
+					wakeup-source;
 				};
 			};