Documentation: pwm-fan: Add description for RPM support

This adds a short description for the new RPM support of the pwm-fan
driver.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>