arm64: dts: imx8mm-evk: add pwm1/backlight support

Add pwm1/backlight support nodes for imx8mm_evk board.
Align with u-boot dts

References:
 - https://patchwork.ozlabs.org/project/uboot/patch/20220326111911.13720-9-tommaso.merciai@amarulasolutions.com/Signed-off-by: Tommaso Merciai <tommaso.merciai@amarulasolutions.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mm-evk.dtsi

@@ -59,6 +59,14 @@ reg_usdhc2_vmmc: regulator-usdhc2 {
 		enable-active-high;
 	};
 
+	backlight: backlight {
+		compatible = "pwm-backlight";
+		pwms = <&pwm1 0 5000000>;
+		brightness-levels = <0 255>;
+		num-interpolated-steps = <255>;
+		default-brightness-level = <250>;
+	};
+
 	ir-receiver {
 		compatible = "gpio-ir-receiver";
 		gpios = <&gpio1 13 GPIO_ACTIVE_LOW>;
@@ -395,6 +403,12 @@ &wdog1 {
 	status = "okay";
 };
 
+&pwm1 {
+	pinctrl-names = "default";
+	pinctrl-0 = <&pinctrl_backlight>;
+	status = "okay";
+};
+
 &iomuxc {
 	pinctrl_fec1: fec1grp {
 		fsl,pins = <
@@ -549,4 +563,10 @@ pinctrl_wdog: wdoggrp {
 			MX8MM_IOMUXC_GPIO1_IO02_WDOG1_WDOG_B	0x166
 		>;
 	};
+
+	pinctrl_backlight: backlightgrp {
+		fsl,pins = <
+			MX8MM_IOMUXC_GPIO1_IO01_PWM1_OUT	0x06
+		>;
+	};
 };