ARM: dts: am335x: replace gpio-key,wakeup with wakeup-source property

Though the keyboard driver for GPIO buttons(gpio-keys) will continue to
check for/support the legacy "gpio-key,wakeup" boolean property to
enable gpio buttons as wakeup source, "wakeup-source" is the new
standard binding.

This patch replaces the legacy "gpio-key,wakeup" with the unified
"wakeup-source" property in order to avoid any futher copy-paste
duplication.

Cc: "Benoît Cousson" <[e-mail]>
Cc: Tony Lindgren <[e-mail]>
Cc: [e-mail]
Signed-off-by: Sudeep Holla <[e-mail]>
Signed-off-by: Tony Lindgren <[e-mail]>

arch/arm/boot/dts/am335x-evm.dts

···
 			label = "volume-up";
 			linux,code = <115>;
 			gpios = <&gpio0 2 GPIO_ACTIVE_LOW>;
-			gpio-key,wakeup;
+			wakeup-source;
 		};
 
 		switch@10 {
 			label = "volume-down";
 			linux,code = <114>;
 			gpios = <&gpio0 3 GPIO_ACTIVE_LOW>;
-			gpio-key,wakeup;
+			wakeup-source;
 		};
 	};
 

arch/arm/boot/dts/am335x-evmsk.dts

···
 			label = "button2";
 			linux,code = <0x102>;
 			gpios = <&gpio0 30 GPIO_ACTIVE_HIGH>;
-			gpio-key,wakeup;
+			wakeup-source;
 		};
 
 		switch@4 {

arch/arm/boot/dts/am335x-pepper.dts

···
 		label = "home";
 		linux,code = <KEY_HOME>;
 		gpios = <&gpio1 22 GPIO_ACTIVE_LOW>;
-		gpio-key,wakeup;
+		wakeup-source;
 	};
 
 	button@1 {
 		label = "menu";
 		linux,code = <KEY_MENU>;
 		gpios = <&gpio1 23 GPIO_ACTIVE_LOW>;
-		gpio-key,wakeup;
+		wakeup-source;
 	};
 
 	buttons@2 {
 		label = "power";
 		linux,code = <KEY_POWER>;
 		gpios = <&gpio0 7 GPIO_ACTIVE_LOW>;
-		gpio-key,wakeup;
+		wakeup-source;
 	};
 };