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>

Documentation/devicetree/bindings/hwmon/adc128d818.txt

+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>;
+	};