board/BuR/brppt2: fix pwms property warning

Fix the following build-time pwms property warning:

w+arch/arm/dts/imx6dl-brppt2.dtb: Warning (pwms_property):
 /backlight:pwms: property size (12) too small for cell size 3

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
diff --git a/arch/arm/dts/imx6dl-brppt2.dts b/arch/arm/dts/imx6dl-brppt2.dts
index 4f1c52b..f515e4c 100644
--- a/arch/arm/dts/imx6dl-brppt2.dts
+++ b/arch/arm/dts/imx6dl-brppt2.dts
@@ -165,6 +165,7 @@
 };
 
 &pwm4 {
+	#pwm-cells = <2>;
 	status = "okay";
 };