ARM: at91/dt: 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: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/sama5d35ek.dts

@@ -49,7 +49,7 @@
 			label = "pb_user1";
 			gpios = <&pioE 27 GPIO_ACTIVE_HIGH>;
 			linux,code = <0x100>;
-			gpio-key,wakeup;
+			wakeup-source;
 		};
 	};
 };