hwmon: (it87) Expose the PWM/temperature mappings

Let the user read the PWM-to-temperature mappings. Until the trip
points are also exposed, this is essentially a way to know how the
BIOS has set things up. The ability to change the settings will be
added later, together with the trip points.

Signed-off-by: Jean Delvare <khali@linux-fr.org>