board: cm_fx6: Enable CONFIG_DM_ETH

Enable CONFIG_DM_ETH to remove compile warning on CM-FX6
SOM based target:

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

Signed-off-by: Suniel Mahesh <sunil.m@techveda.org>