dt-bindings: hwmon: Add optional regulator support to pwm-fan

This adds an optional regulator support (e.g. switchable supply) to the
pwm fan binding.
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/pwm-fan.txt

@@ -6,6 +6,9 @@ Required properties:
 - cooling-levels      : PWM duty cycle values in a range from 0 to 255
 			which correspond to thermal cooling states
 
+Optional properties:
+- fan-supply    : phandle to the regulator that provides power to the fan
+
 Example:
 	fan0: pwm-fan {
 		compatible = "pwm-fan";