kontron-sl-mx6ul: switch to DM_SERIAL

Enable DM_SERIAL to avoid board removal warning.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
1 file changed