dt-bindings: hwmon: amc6821: Add cooling levels

The fan can be used as a cooling device, add a description of the
`cooling-levels` property and restrict the maximum value to 255, which
is the highest PWM duty cycle supported by the AMC6821 fan controller.

Signed-off-by: João Paulo Gonçalves <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Guenter Roeck <[e-mail]>

Documentation/devicetree/bindings/hwmon/ti,amc6821.yaml

···
     $ref: fan-common.yaml#
     unevaluatedProperties: false
 
+    properties:
+      cooling-levels:
+        description: PWM duty cycle values corresponding to thermal cooling states.
+        items:
+          maximum: 255
+
   "#pwm-cells":
     const: 2
     description: |