net: fec_mxc: Remove non-DM_ETH code

Now that all boards have been converted, remove the non-DM_ETH code.

Cc: Fabio Estevam <festevam@gmail.com>
Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>