net: dsa: mv88e6171: Add support for hardware bridging

Wire up the common code for setting up hardware bridging
and access to the forwarding database.

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