dt-bindings: hwmon: Add max6639

Add Devicetree binding documentation for Maxim MAX6639 temperature
monitor with PWM fan-speed controller.

Signed-off-by: Naresh Solanki <naresh.solanki@9elements.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240528125122.1129986-1-naresh.solanki@9elements.com


[groeck: Dropped unnecessary '|' after description:]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>