MIPS: sfr_nb4-ser_ram: migrate USB driver to driver model
Build warnings:
+===================== 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.txt for more info.
+====================================================