net: dsa: mv88e6xxx: Add Hardware bridging support

Bridge support is similar for all chips supported by the mv88e6xxx code,
so add the code there.

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