imx: imx8mq-cm: enable CONFIG_DM_SERIAL

Marked related nodes as u-boot,dm-spl for serial driver model
Enable CONFIG_DM_SERIAL

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>

arch/arm/dts/imx8mq-cm-u-boot.dtsi

@@ -9,6 +9,14 @@
 	};
 };
 
+&pinctrl_uart1 {
+	u-boot,dm-spl;
+};
+
+&uart1 {
+	u-boot,dm-spl;
+};
+
 &binman {
 	 u-boot-spl-ddr {
 		filename = "u-boot-spl-ddr.bin";