[D18] add uart4 support

arch/arm64/boot/dts/seco/seco-imx8mp-d18.dts

@@ -499,6 +499,15 @@
 	status = "okay";
 };
 
+&uart4 {
+	pinctrl-names = "default";
+	pinctrl-0 = <&pinctrl_uart4>;
+	assigned-clocks = <&clk IMX8MP_CLK_UART4>;
+	assigned-clock-parents = <&clk IMX8MP_SYS_PLL1_80M>;
+	fsl,uart-has-rtscts;
+	status = "okay";
+};
+
 &usb3_phy0 {
 	status = "okay";
 };
@@ -796,6 +805,13 @@
 		>;
 	};
 
+	pinctrl_uart4: uart4grp {
+		fsl,pins = <
+			MX8MP_IOMUXC_UART4_RXD__UART4_DCE_RX	0x49
+			MX8MP_IOMUXC_UART4_TXD__UART4_DCE_TX	0x49
+		>;
+	};
+
 	pinctrl_usb1_vbus: usb1grp {
 		fsl,pins = <
 			MX8MP_IOMUXC_GPIO1_IO14__USB2_PWR		0x19