mlxsw: spectrum: Add port to linecard mapping

For each port get slot_index using PMLP register. For ports residing
on a linecard, identify it with the linecard by setting mapping
using devlink_port_linecard_set() helper. Use linecard slot index for
PMTDB register queries.

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