imx: imx8m[m/n]_beacon: Enable SPL_DM_SERIAL

Enable CONFIG_SPL_DM_SERIAL. uart2 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>
Tested-by: Adam Ford <aford173@gmail.com> #imx8mm_beacon
Reviewed-by: Fabio Estevam <festevam@denx.de>
Tested-by: Adam Ford <aford173@gmail.com> #imx8mn_beacon
7 files changed