ARM: dts: imx6dl-aristainetos_7: Use #pwm-cells = <3> for imx27-pwm device

The binding dictates using 3 pwm-cells. Adhere to that.

This fixes the following dtbs_check warning:

	arch/arm/boot/dts/nxp/imx/imx6dl-aristainetos_7.dtb: pwm@2088000: #pwm-cells:0:0: 3 was expected
		from schema : http://devicetree.org/schemas/pwm/imx-pwm.yaml#Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/nxp/imx/imx6dl-aristainetos_7.dts

@@ -49,7 +49,7 @@ display0_in: endpoint {
 
 	backlight {
 		compatible = "pwm-backlight";
-		pwms = <&pwm3 0 3000>;
+		pwms = <&pwm3 0 3000 0>;
 		brightness-levels = <0 4 8 16 32 64 128 255>;
 		default-brightness-level = <6>;
 		pinctrl-names = "default";
@@ -69,6 +69,5 @@ &ipu1_di0_disp0 {
 };
 
 &pwm3 {
-	#pwm-cells = <2>;
 	status = "okay";
 };