imx: imx8mq-pico: 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>
diff --git a/configs/pico-imx8mq_defconfig b/configs/pico-imx8mq_defconfig
index 1b0d12c..91aca29 100644
--- a/configs/pico-imx8mq_defconfig
+++ b/configs/pico-imx8mq_defconfig
@@ -83,5 +83,6 @@
 CONFIG_DM_REGULATOR_GPIO=y
 CONFIG_SPL_POWER_I2C=y
 CONFIG_DM_RESET=y
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_DM_THERMAL=y