i8k: Implement hwmon based fan speed control

Fan speed can be set to off, slow, and fast, which can be exported
to userspace through the hwmon ABI as pwm1 / pwm2.

Cc: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>