imx8mm-kontron-n801x-s-lvds: fix pwms property warnings

Fix the following build-time pwms property warnings:

w+arch/arm/dts/imx8mm-kontron-n801x-s-lvds.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/imx8mm-kontron-n801x-s-lvds.dts b/arch/arm/dts/imx8mm-kontron-n801x-s-lvds.dts
index dd1adde..40c1473 100644
--- a/arch/arm/dts/imx8mm-kontron-n801x-s-lvds.dts
+++ b/arch/arm/dts/imx8mm-kontron-n801x-s-lvds.dts
@@ -87,6 +87,7 @@
 &pwm1 {
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_pwm1>;
+	#pwm-cells = <2>;
 	status = "okay";
 };