hwmon: Add pwm-fan driver

The pwm-fan driver enables control of fans connected to PWM lines.
This driver uses the PWM framework, so it is compatible with all
PWM devices that provide drivers through the PWM framework.

Signed-off-by: Kamil Debski <k.debski@samsung.com>
Reviewed-by: Varka Bhadram <varkabhadram@gmail.com>
Reviewed-by: Tobias Klauser <tklauser@distanz.ch>
[Guenter Roeck: Last argument to devm_of_pwm_get is pointer, use NULL]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>