arm: mvebu: turris_mox: Use DM registered MDIO

In order to be able to get rid of the non-DM MDIO bus registered in
mvneta driver, start using the DM registered one in Turris MOX board
code.

This also allows us to drop the hack introduced in MOX' -u-boot.dtsi
file.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>