configs: ls208x: enable DM_SERIAL

Now that the DT nodes for the serial devices are in place for these
boards, enable DM_SERIAL in the associated configs.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
3 files changed