imx: imx8mm-cl-iot-gate: Enable DM_SERIAL

Enable CONFIG_DM_SERIAL. uart3 and its pinmux was already
marked with u-boot,dm-spl.
Move preloader_console_init after spl_early_init to make sure driver
model work.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
diff --git a/configs/imx8mm-cl-iot-gate_defconfig b/configs/imx8mm-cl-iot-gate_defconfig
index 67f7576..dae7ddc 100644
--- a/configs/imx8mm-cl-iot-gate_defconfig
+++ b/configs/imx8mm-cl-iot-gate_defconfig
@@ -124,6 +124,7 @@
 CONFIG_DM_REGULATOR_GPIO=y
 CONFIG_DM_RTC=y
 CONFIG_RTC_ABX80X=y
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_SPI=y
 CONFIG_DM_SPI=y