MIPS: DTS: img: Fix schema warnings for pwm-leds

The node names for devices using the pwm-leds driver follow a certain
naming scheme (now).  Parent node name is not enforced, but recommended
by DT project.
Signed-off-by: NAlexander Dahl <post@lespocky.de>
Signed-off-by: NThomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/dts/img/pistachio_marduk.dts

@@ -46,9 +46,10 @@
 		regulator-max-microvolt = <1800000>;
 	};
 
-	leds {
+	led-controller {
 		compatible = "pwm-leds";
-		heartbeat {
+
+		led-1 {
 			label = "marduk:red:heartbeat";
 			pwms = <&pwm 3 300000>;
 			max-brightness = <255>;