net: dsa: realtek: add new mdio interface for drivers

This driver is a mdio_driver instead of a platform driver (like
realtek-smi).

ds_ops was duplicated for smi and mdio usage as mdio interfaces uses
phy_{read,write} in ds_ops and the presence of phy_read is incompatible
with external slave_mii_bus allocation.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Tested-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: David S. Miller <davem@davemloft.net>