mlxsw: spectrum_cnt: Expose devlink resource occupancy for counters

Implement occupancy counting for counters and expose over devlink
resource API.

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