hwmon: add generic GPIO fan driver

This patch adds hwmon support for fans connected to GPIO lines.

Platform specific information such as GPIO pinout and speed conversion array
(rpm from/to GPIO value) are passed to the driver via platform_data.

Signed-off-by: Simon Guinot <sguinot@lacie.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>