ARM: at91/dt: sama5d3: enable D2 as the heartbeat LED

This D2 led is available for all sama5d3x-ek board. So make it a
heartbeat LED.
Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/sama5d3xcm.dtsi

@@ -122,6 +122,7 @@ leds {
 		d2 {
 			label = "d2";
 			gpios = <&pioE 25 GPIO_ACTIVE_LOW>;	/* PE25, conflicts with A25, RXD2 */
+			linux,default-trigger = "heartbeat";
 		};
 	};
 };