net: fec: add support for DM_MDIO

net: fec: add support for DM_MDIO

Add support for DM_MDIO by registering a UCLASS_MDIO driver and
attempting to use it. This is necessary if wanting to use a DSA
driver for example hanging off of the FEC MAC.

Care is taken to fallback to non DM_MDIO mii bus as several boards define
DM_MDIO without having the proper device-tree configuration necessary
such as an mdio subnode, a phy-mode prop, and either a valid phy-handle
prop or fixed-phy subnode which will cause dm_eth_phy_connect() to fail.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>