MIPS: ci20_mmc: migrate ethernet driver to driver model
Build warnings:
+===================== WARNING ======================
+This board does not use CONFIG_DM_ETH (Driver Model
+for Ethernet drivers). Please update the board to use
+CONFIG_DM_ETH before the v2020.07 release. Failure to
+update by the deadline may result in board removal.
+See doc/driver-model/migration.rst for more info.
+====================================================