imx: imx8mp_rsb3720a1: convert to DM_SERIAL

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

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>