net/mlx5: Embed struct devlink_port into driver structure

Struct devlink_port is usually embedded in a driver-specific struct
which allows to carry driver context to devlink port ops.

Introduce a container struct to include devlink_port struct
in preparation to also include driver context for devlink port ops.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>