configs: arm: ls1021aiot: enable CONFIG_DM_USB support

configs: arm: ls1021aiot: enable CONFIG_DM_USB support

Enable CONFIG_DM_USB to remove below compile warning:

===================== WARNING ======================
This board does not use CONFIG_DM_USB. Please update
the board to use CONFIG_DM_USB before the v2019.07 release.
Failure to update by the deadline may result in board removal.
See doc/driver-model/migration.rst for more info.
====================================================

Signed-off-by: Ran Wang <ran.wang_1@nxp.com>
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>