mlxsw: reg: Extend MTMP register with new slot number field

Extend MTMP (Management Temperature Register) with new field specifying
the slot index. The purpose of this field is to support access to MTMP
register for reading temperature sensors on modular systems.
For non-modular systems the 'sensor_index' uniquely identifies the cage
sensors, while 'slot_index' is always 0. For modular systems the
sensors are identified by:
- 'slot_index', specifying the slot index, where line card is located;
- 'sensor_index', specifying cage sensor within the line card.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>