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>
4 files changed