mlxsw: spectrum_cnt: Expose subpool sizes over devlink resources

Implement devlink resources support for counter pools. Move the subpool
sizes calculations into the new resources register function.

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>