mlxsw: Extend PGT APIs to support maintaining list of ports per entry

Add an API to associate a PGT entry with SMPE index and add or remove a
port. This API will be used by FID code and MDB code, to add/remove port
from specific PGT entry.

When the first port is added to PGT entry, allocate the entry in the given
MID index, when the last port is removed from PGT entry, free it.

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