mlxsw: spectrum_policer: Add devlink resource support

Expose via devlink-resource the maximum number of single-rate policers
and their current occupancy. Example:

$ devlink resource show pci/0000:01:00.0
...
  name global_policers size 1000 unit entry dpipe_tables none
    resources:
      name single_rate_policers size 968 occ 0 unit entry dpipe_tables none

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>