arm: mvebu: add button for OpenBlocks AX3-4

The OpenBlocks AX3-4 board has one software-controlled button on the
front side, labeled "INIT", so we add minimal support for this button
in the kernel.
Signed-off-by: NThomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: NJason Cooper <jason@lakedaemon.net>

arch/arm/boot/dts/armada-xp-openblocks-ax3-4.dts

@@ -66,6 +66,18 @@
 			};
 		};
 
+		gpio_keys {
+			compatible = "gpio-keys";
+			#address-cells = <1>;
+			#size-cells = <0>;
+
+			button@1 {
+				label = "Init Button";
+				linux,code = <116>;
+				gpios = <&gpio1 28 0>;
+			};
+		};
+
 		mdio {
 			phy0: ethernet-phy@0 {
 				reg = <0>;