net/mlx5e: Add trace point for neigh update

Allow tracing neigh state during neigh update task that is executed on
workqueue and is scheduled by neigh state change event.

Usage example:
 ># cd /sys/kernel/debug/tracing
 ># echo mlx5:mlx5e_rep_neigh_update >> set_event
 ># cat trace
    ...
    kworker/u48:7-2221  [009] ...1  1475.387435: mlx5e_rep_neigh_update:
netdev: ens1f0 MAC: 24:8a:07:9a:17:9a IPv4: 1.1.1.10 IPv6: ::ffff:1.1.1.10 neigh_connected=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>