hwmon: (pwm-fan) Make SENSORS_PWM_FAN depend on OF

This driver won't instantiate if OF is not configured, thus add OF dependency.
Also add COMPILE_TEST to increase build testing coverage.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>