net: dsa: ocelot: seville: utilize of_mdiobus_register

Switch seville to use of_mdiobus_register(bus, NULL) instead of just
mdiobus_register. This code is about to be pulled into a separate module
that can optionally define ports by the device_node.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>