board: LED id for nrf52840_pca10056

Add LED ID for nrf52840_pca10056 board.
This id is required for gpio leds driver.

Signed-off-by: Kamil Gawor <Kamil.Gawor@nordicsemi.no>

Author: Kamil Gawor

boards/arm/nrf52840dk_nrf52840/nrf52840dk_nrf52840.dts

@@ -27,18 +27,22 @@
 		led0: led_0 {
 			gpios = <&gpio0 13 GPIO_ACTIVE_LOW>;
 			label = "Green LED 0";
+			id = <1>;
 		};
 		led1: led_1 {
 			gpios = <&gpio0 14 GPIO_ACTIVE_LOW>;
 			label = "Green LED 1";
+			id = <2>;
 		};
 		led2: led_2 {
 			gpios = <&gpio0 15 GPIO_ACTIVE_LOW>;
 			label = "Green LED 2";
+			id = <3>;
 		};
 		led3: led_3 {
 			gpios = <&gpio0 16 GPIO_ACTIVE_LOW>;
 			label = "Green LED 3";
+			id = <4>;
 		};
 	};