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>

configs/cm_fx6_defconfig

@@ -84,6 +84,7 @@ CONFIG_DM_PMIC=y
 CONFIG_DM_REGULATOR=y
 CONFIG_SPI=y
 CONFIG_MXC_SPI=y
+CONFIG_DM_ETH=y
 CONFIG_USB=y
 CONFIG_DM_USB=y
 CONFIG_USB_KEYBOARD=y