mlxsw: Adding layer 2 multicast support

Add SWITCHDEV_OBJ_ID_PORT_MDB switchdev ops support. On first MDB insertion
creates a new multicast group (MID) and add members port to the MID. Also
add new MDB entry for the flooding-domain (fid-vid) and link the MDB entry
to the newly constructed MC group.

Signed-off-by: Elad Raz <eladr@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>