imx6: Remove unneeded CONFIG_DM_MDIO

As explained in the CONFIG_DM_MDIO text inside drivers/net/Kconfig:

"Useful in particular for systems that support
DM_ETH and have a stand-alone MDIO hardware block shared by multiple
Ethernet interfaces."

i.MX6 has a single FEC instance, so there is no need to select
CONFIG_DM_MDIO.

Remove it from the i.MX6 defconfig files.

Signed-off-by: Fabio Estevam <festevam@gmail.com>