hwmon: (it87) Add support for the ITE IT8720F

Allow it87.c to handle IT8720 chipset like IT8718 in order to
retrieve voltage, temperatures and fans speed from sensors
tools. Also updating the related documentation.

Signed-off-by: Jean-Marc Spaggiari <jean-marc@spaggiari.org>
Signed-off-by: Jean Delvare <khali@linux-fr.org>