HWMON: w83791d: New hardware monitoring driver for the Winbond W83791D

Add support for the w83791d sensor chip. The w83791d hardware is
somewhere between the w83781d and the w83792d and this driver code
is derived from the code that supports those chips.

Signed-off-by: Charles Spirakis <bezaur@gmail.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>