imx8mp-rsb3720-a1: fix pwms property warnings

Fix the following build-time pwms property warnings:

w+arch/arm/dts/imx8mp-rsb3720-a1.dtb: Warning (pwms_property):
 /lvds_backlight@0:pwms: property size (12) too small for cell size 3
w+arch/arm/dts/imx8mp-rsb3720-a1.dtb: Warning (pwms_property):
 /lvds_backlight@1: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/imx8mp-rsb3720-a1.dts b/arch/arm/dts/imx8mp-rsb3720-a1.dts
index 1ef1c0c..2d8dc19 100644
--- a/arch/arm/dts/imx8mp-rsb3720-a1.dts
+++ b/arch/arm/dts/imx8mp-rsb3720-a1.dts
@@ -143,12 +143,14 @@
 &pwm2 {
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_pwm2>;
+	#pwm-cells = <2>;
 	status = "okay";
 };
 
 &pwm3 {
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_pwm3>;
+	#pwm-cells = <2>;
 	status = "okay";
 };