net: dsa: OF-ware slave_mii_bus

If found, register the DSA internally allocated slave_mii_bus with an OF
"mdio" child object. It can save some drivers from creating their
custom internal MDIO bus.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>