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

Introduce extension of mlxsw_afa_ops in order to add/del mirroring and
implement the ops for Spectrum.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>