net/mlx5e: Add trace point for neigh used value update

Allow tracing result of neigh used value update task that is executed
periodically on workqueue.

Usage example:
 ># cd /sys/kernel/debug/tracing
 ># echo mlx5:mlx5e_tc_update_neigh_used_value >> set_event
 ># cat trace
    ...
    kworker/u48:4-8806  [009] ...1 55117.882428: mlx5e_tc_update_neigh_used_value:
netdev: ens1f0 IPv4: 1.1.1.10 IPv6: ::ffff:1.1.1.10 neigh_used=1

Added corresponding documentation in
    Documentation/networking/device-driver/mellanox/mlx5.rst

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Reviewed-by: Dmytro Linkin <dmitrolin@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>