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>
3 files changed