mlxsw: Enable configuration of flooding domains

As part of the introduction of L2 offloads, allow different ports to
join/leave the flooding domain, according to user configuration.

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