imx6: aristainetos: fix pwms property warnings

Fix the following build-time pwms property warnings:

w+arch/arm/dts/imx6dl-aristainetos2c_7.dtb: Warning (pwms_property):
 /backlight:pwms: property size (12) too small for cell size 3
w+arch/arm/dts/imx6dl-aristainetos2c_cslb_7.dtb: Warning (pwms_property):
 /backlight:pwms: property size (12) too small for cell size 3

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
diff --git a/arch/arm/dts/imx6qdl-aristainetos2-common.dtsi b/arch/arm/dts/imx6qdl-aristainetos2-common.dtsi
index 5701436..dd683dc 100644
--- a/arch/arm/dts/imx6qdl-aristainetos2-common.dtsi
+++ b/arch/arm/dts/imx6qdl-aristainetos2-common.dtsi
@@ -344,6 +344,7 @@
 };
 
 &pwm1 {
+	#pwm-cells = <2>;
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_pwm1>;
 	status = "okay";