dt-bindings: hwmon: gpio-fan: Add optional regulator support

This adds an optional regulator support (e.g. switchable supply) to the
GPIO fan binding.

Signed-off-by: Alexander Stein <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[groeck: Changed power supply description as suggested by Krzysztof]
Signed-off-by: Guenter Roeck <[e-mail]>

Documentation/devicetree/bindings/hwmon/gpio-fan.yaml

···
   alarm-gpios:
     maxItems: 1
 
+  fan-supply:
+    description: Power supply for fan
+
   gpio-fan,speed-map:
     $ref: /schemas/types.yaml#/definitions/uint32-matrix
     minItems: 2