net: dsa: mv88e6171: Enable access to phys via internal mdio bus

When the device is configured to use single chip addressing mode, the
phy devices of the port are not accessible on the host MDIO
bus. Instead the switch internal MDIO bus must be used. For this to
work, the phy polling unit must be enabled.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>