configs: ls1046ardb: enable DM_SERIAL

As the serial devices are configured in the device tree, enable
DM_SERIAL in the ls1046ardb defconfigs.

Signed-off-by: Camelia Groza <camelia.groza@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
8 files changed