net: introduce MDIO DM class for MDIO devices

Adds UCLASS_MDIO DM class supporting MDIO buses that are probed as
stand-alone devices.  Useful in particular for systems that support
DM_ETH and have a stand-alone MDIO hardware block shared by multiple
Ethernet interfaces.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>