mlxsw: Add an indication of SMPE index validity for PGT table

In Spectrum-1, the index into the MPE table - called switch multicast to
port egress VID (SMPE) - is derived from the PGT entry, whereas in
Spectrum-2 and later ASICs it is derived from the FID.

Therefore, in Spectrum-1, the SMPE index needs to be programmed as part of
the PGT entry via SMID register, while it is reserved for Spectrum-2 and
later ASICs.

Add 'pgt_smpe_index_valid' boolean as part of 'struct mlxsw_sp' and set
it to true for Spectrum-1 and to false for the later ASICs. Add
'smpe_index_valid' as part of 'struct mlxsw_sp_pgt' and set it according
to the value in 'struct mlxsw_sp' as part of PGT initialization.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>