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 <joao.goncalves@toradex.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250613-b4-amc6821-cooling-device-support-v4-1-a8fc063c55de@toradex.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>