dt-bindings: hwmon: add dps650ab documentation

Bindings for Delta Electronics DPS-650-AB power supply.

Signed-off-by: Xiaoting Liu <xiaoting.liu@hxt-semitech.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>