net/mlx5: Add vnic devlink health reporter to PFs/VFs

Create a vnic devlink health reporter for PFs/VFs interfaces.
The reporter's diagnose callback displays the values of vNIC/vport
transport debug counters of PFs/VFs, as follows:

$ devlink health diagnose pci/0000:08:00.0 reporter vnic
 vNIC env counters:
    total_error_queues: 0 send_queue_priority_update_flow: 0
    comp_eq_overrun: 0 async_eq_overrun: 0 cq_overrun: 0
    invalid_command: 0 quota_exceeded_command: 0
    nic_receive_steering_discard: 0

Moreover, add documentation on the reporter functionality and the
counters description.

While at it, expose the vNIC counters diagnose function to be used by
the downstream patch, which will reveal the counters for representor
interfaces.

Signed-off-by: Maher Sanalla <msanalla@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>