mlxsw: reg: Extend SBPM register for occupancy control

Since it is not possible to get and clear Port-Pool occupancy data using
SBSR register, there's a need to implement that using SBPM.
Extend pack helper and add unpack helper to get 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>