ARM: dts: imx6: RDU2: manage backlight from panel

Now that the backlight driver is upstream, we can properly manage the
backlight from the panel.
Signed-off-by: NLucas Stach <l.stach@pengutronix.de>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6qdl-zii-rdu2.dtsi

@@ -210,6 +210,7 @@
 
 	panel {
 		power-supply = <&reg_3p3v_display>;
+		backlight = <&sp_backlight>;
 		status = "disabled";
 
 		port {
@@ -327,7 +328,7 @@
 			compatible = "zii,rave-sp-watchdog";
 		};
 
-		backlight {
+		sp_backlight: backlight {
 			compatible = "zii,rave-sp-backlight";
 		};