hwmon: (pc87427) Add support for manual fan speed control

Add initial support for PWM outputs of the PC87427 Super-I/O chip.
Only mode change and manual fan speed control are supported. Automatic
mode configuration isn't supported, and won't be until at least one
board is known, which makes uses of the PWM outputs.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Guenter Roeck <guenter.roeck@ericsson.com>