mlxsw: spectrum_buffers: Implement occupancy monitoring

Implement occupancy API introduced in devlink and mlxsw core. This is
done by accessing SBPM register for Port-Pool and SBSR for Port-TC
current and max occupancy values. Max clear is implemented using the
same registers.

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