mlxsw: spectrum: Extend mlxsw_afa_ops for counter index and implement for Spectrum

Introduce extension of mlxsw_afa_ops in order to get/put counter indexes
and implement the ops for Spectrum.

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