ARM: dts: at91: replace gpio-key,wakeup with wakeup-source for sam9260ek

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 further copy-paste
duplication.
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

arch/arm/boot/dts/at91sam9260ek.dts

@@ -174,14 +174,14 @@ btn3 {
 			label = "Button 3";
 			gpios = <&pioA 30 GPIO_ACTIVE_LOW>;
 			linux,code = <0x103>;
-			gpio-key,wakeup;
+			wakeup-source;
 		};
 
 		btn4 {
 			label = "Button 4";
 			gpios = <&pioA 31 GPIO_ACTIVE_LOW>;
 			linux,code = <0x104>;
-			gpio-key,wakeup;
+			wakeup-source;
 		};
 	};