configs: T1024RDB: enable DM_SERIAL

As the serial devices are configured in the device tree, enable
DM_SERIAL in the non-SPL T1024RDB defconfigs.

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