imx: Convert some boards to DM_ETH

A small number of i.MX6/7 and vf610 boards have not enabled DM_ETH yet.
Given the state of the rest of the platform, enable DM_ETH.

Cc: Alison Wang <alison.wang@nxp.com>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Richard Hu <richard.hu@technexion.com>
Cc: Troy Kisky <troy.kisky@boundarydevices.com>
Cc: Uri Mashiach <uri.mashiach@compulab.co.il>
Signed-off-by: Tom Rini <trini@konsulko.com>
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I would very much welcome tested and perhaps slightly more complete
migrations for these platforms as the deadline passed with v2020.07.
But, in order to do more Kconfig migrations as well, I did this quick
pass.