imx: imx8mn_evk: enable CONFIG_DM_SERIAL

Enable CONFIG_DM_SERIAL. uart2 and its pinmux was already
marked with u-boot,dm-spl.

move the preloader_console_init() call after spl_init() to
avoid board hang

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/configs/imx8mn_ddr4_evk_defconfig b/configs/imx8mn_ddr4_evk_defconfig
index 917cdb5..474d9cf 100644
--- a/configs/imx8mn_ddr4_evk_defconfig
+++ b/configs/imx8mn_ddr4_evk_defconfig
@@ -76,6 +76,7 @@
 CONFIG_DM_REGULATOR=y
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_SYSRESET=y
 CONFIG_SPL_SYSRESET=y