net/mlx5: E-Switch, Create generic header struct to be used by representors

Now that we don't store type dependent data in struct mlx5_eswitch_rep
we can create a generic interface, and representor type.

struct mlx5_eswitch_rep will store an array of interfaces, each
interface is used by a different representor type.

Once we moved to a more generic interface, rdma driver representors can
be added and utilize the same mechanism as the Ethernet driver
representors use.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>