imx: imx8mn_var_som: enable DM_SERIAL

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

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
diff --git a/configs/imx8mn_var_som_defconfig b/configs/imx8mn_var_som_defconfig
index 898f3f2..889bcf7 100644
--- a/configs/imx8mn_var_som_defconfig
+++ b/configs/imx8mn_var_som_defconfig
@@ -96,6 +96,7 @@
 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_SYSRESET_PSCI=y