mx6slevk: Convert to DM_ETH

This fixes the following warning:

===================== 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.
====================================================

Signed-off-by: Pedro Jardim <jardim.c.pedro@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>