net/mlx4: add flow support for multicast traffic

Give users the ability to create flow rules that match all multicast
traffic. Like promiscuous flow rules, they come with restrictions such as
not allowing additional matching criteria.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>