net/mlx5e: Create single netdev per SD group

Integrate the SD library calls into the auxiliary_driver ops in
preparation for creating a single netdev for the multiple devices
belonging to the same SD group.

SD is still disabled at this stage. It is enabled by a downstream patch
when all needed parts are implemented.

The netdev is created only when the SD group, with all its participants,
are ready. It is later destroyed if any of the participating devices
drops.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>