arm64: dts: imx8mn-evk: Add UART3 support

UART3 pins are available in the J1003 connector.

Add support for it.
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mn-evk.dtsi

@@ -212,6 +212,15 @@ &uart2 { /* console */
 	status = "okay";
 };
 
+&uart3 {
+	pinctrl-names = "default";
+	pinctrl-0 = <&pinctrl_uart3>;
+	assigned-clocks = <&clk IMX8MN_CLK_UART3>;
+	assigned-clock-parents = <&clk IMX8MN_SYS_PLL1_80M>;
+	uart-has-rtscts;
+	status = "okay";
+};
+
 &usbotg1 {
 	dr_mode = "otg";
 	hnp-disable;
@@ -374,6 +383,15 @@ MX8MN_IOMUXC_UART2_TXD_UART2_DCE_TX	0x140
 		>;
 	};
 
+	pinctrl_uart3: uart3grp {
+		fsl,pins = <
+			MX8MN_IOMUXC_ECSPI1_SCLK_UART3_DCE_RX		0x140
+			MX8MN_IOMUXC_ECSPI1_MOSI_UART3_DCE_TX		0x140
+			MX8MN_IOMUXC_ECSPI1_SS0_UART3_DCE_RTS_B	0x140
+			MX8MN_IOMUXC_ECSPI1_MISO_UART3_DCE_CTS_B	0x140
+		>;
+	};
+
 	pinctrl_usdhc2_gpio: usdhc2gpiogrp {
 		fsl,pins = <
 			MX8MN_IOMUXC_GPIO1_IO15_GPIO1_IO15	0x1c4