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

Extend MTBR (Management Temperature Bulk Register) with new field
specifying the slot number. The purpose of this field is to support
access to MTBR register for reading temperature sensors on modular
system. For non-modular systems the 'sensor_index' uniquely identifies
the cage sensors. For modular systems the sensors are identified by two
indexes:
- 'slot_index', specifying the slot number, 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>