devlink: implement shared buffer occupancy monitoring interface

User needs to monitor shared buffer occupancy. For that, he issues a
snapshot command in order to instruct hardware to catch current and
maximal occupancy values, and clear command in order to clear the
historical maximal values.

Also port-pool and tc-pool-bind command response messages are extended to
carry occupancy values.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>