ARM: dts: imx6ul-evk: Pass the 'backlight' property

Pass the 'backlight' property as described in
Documentation/devicetree/bindings/display/panel/simple-panel.txt
Signed-off-by: NMarco Franchi <marco.franchi@nxp.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6ul-14x14-evk.dts

@@ -22,7 +22,7 @@
 		reg = <0x80000000 0x20000000>;
 	};
 
-	backlight {
+	backlight_display: backlight-display {
 		compatible = "pwm-backlight";
 		pwms = <&pwm1 0 5000000>;
 		brightness-levels = <0 4 8 16 32 64 128 255>;
@@ -81,6 +81,7 @@
 
 	panel {
 		compatible = "innolux,at043tn24";
+		backlight = <&backlight_display>;
 
 		port {
 			panel_in: endpoint {