ARM: dts: am335x-pcm953: Fix legacy wakeup source binding

Replaced the legacy binding "gpio-key,wakeup" with "wakeup-source" as
noted in the kernel documentation.
Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Reported-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/boot/dts/am335x-pcm-953.dtsi

@@ -63,14 +63,14 @@ button@0 {
 			label = "home";
 			linux,code = <KEY_HOME>;
 			gpios = <&gpio3 7 GPIO_ACTIVE_HIGH>;
-			gpio-key,wakeup;
+			wakeup-source;
 		};
 
 		button@1 {
 			label = "menu";
 			linux,code = <KEY_MENU>;
 			gpios = <&gpio3 8 GPIO_ACTIVE_HIGH>;
-			gpio-key,wakeup;
+			wakeup-source;
 		};
 
 	};