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

Extend MCIA (Management Cable Info Access Register) with new field
specifying the slot number. The purpose of this field is to support
access to MCIA register for reading cage cable information on modular
system. For non-modular systems the 'module' number uniquely identifies
the transceiver location. For modular systems the transceivers are
identified by two indexes:
- 'slot_index', specifying the slot number, where line card is located;
- 'module', specifying cage transceiver 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>