net: dsa: mv88e6xxx: Add support for fdb_add, fdb_del, and fdb_getnext

No vlan support at this time.

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>