mlxsw: core: Extend hwmon interface with inter-connect temperature attributes

Add new attributes to hwmon object for exposing inter-connects temperature
input, highest, reset_history temperatures and label. Temperatures are read
from Management Temperature Register.
The number of inter-connect devices is read from Management General
Peripheral Information Register.

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>