mlxsw: reg: Extend sensor index field size of Management Temperature Register

Extend the size of sensor_index field of MTMP (Management Temperature
Register), from 8 to 12 bits due to hardware change.
Add define for sensor index for Gear Box (inter-connects) temperature
reading.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>