net/mlx5e: Add vnic devlink health reporter to representors

Create a new devlink health reporter for representor interface, which
reports the values of representor vnic diagnostic counters when diagnosed.

This patch will allow admins to monitor VF diagnostic counters through
the representor-interface vnic reporter.

Example of usage:
$ devlink health diagnose pci/0000:08:00.0/65537 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

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