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 <alexander.stein@ew.tq-group.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250210145934.761280-2-alexander.stein@ew.tq-group.com


[groeck: Changed power supply description as suggested by Krzysztof]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>