mlxsw: reg: Add Switch Multicast Port to Egress VID Register

The SMPE register maps {egress_port, SMPE index} -> VID.

The device includes two main tables to support layer 2 multicast (i.e.,
MDB and flooding). These are the PGT (Port Group Table) and the
MPE (Multicast Port Egress) table.
- PGT is {MID -> (bitmap of local_port, SPME index)}
- MPE is {(Local port, SMPE index) -> eVID}

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.

In the legacy model, software did not interact with this table as it was
completely hidden in firmware. In the new model, software needs to
populate the table itself in order to map from {Local port, SMPE index} to
an egress VID. This is done using the SMPE register.

Add the register for future use.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>