net/mlx5: Add monitor commands layout and event data

Will be used in downstream patch to monitor counter changes
by the HCA and report it to the driver by an event.
The driver will update its counters cached data accordingly.

Signed-off-by: Eyal Davidovich <eyald@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>