devicetree: hwmon: Add bindings for ADC128D818

TI ADC128D818 ADC System Monitor With Temperature Sensor

--------------------------------------------------------

Operation modes:

 - Mode 0:  7 single-ended voltage readings (IN0-IN6),

            1 temperature reading (internal)

 - Mode 1:  8 single-ended voltage readings (IN0-IN7),

            no temperature

 - Mode 2:  4 pseudo-differential voltage readings

              (IN0-IN1, IN3-IN2, IN4-IN5, IN7-IN6),

            1 temperature reading (internal)

 - Mode 3:  4 single-ended voltage readings (IN0-IN3),

            2 pseudo-differential voltage readings

              (IN4-IN5, IN7-IN6),

            1 temperature reading (internal)

If no operation mode is configured via device tree, the driver keeps the

currently active chip operation mode (default is mode 0).

Required node properties:

 - compatible:  must be set to "ti,adc128d818"

 - reg:         I2C address of the device

Optional node properties:

 - ti,mode:     Operation mode (see above).

Example (operation mode 2):

	adc128d818@1d {

		compatible = "ti,adc128d818";

		reg = <0x1d>;

		ti,mode = <2>;

	};