mlxsw: spectrum: Implement FDB add/remove/dump for LAG

Implement FDB offloading for lagged ports, including learning LAG FDB
entries, adding/removing static FDB entries and dumping existing LAG FDB
entries.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>