devicetree: hwmon: Add bindings for ADC128D818

Add bindings documentation for the ADC128D818 driver, featuring default I2C
properties along with the optional 'mode' property for chip operation mode
selection (see datasheet, sec. 8.4.1).

Signed-off-by: Alexander Koch <mail@alexanderkoch.net>
Acked-by: Michael Hornung <mhornung.linux@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
[groeck: Fixed whitespace error]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>