hwmon: (f71882fg) Add PWM support

Add PWM (fan speed control) support to the f71882fg driver. Both
manual control and automatic (temperature-based) modes are supported.
Additionally, each mode has a PWM-based and an RPM-based variant. By
default we use the mode set by the BIOS.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Cc: Mark van Doesburg <mark.vandoesburg@hetnet.nl>
Signed-off-by: Jean Delvare <khali@linux-fr.org>