iio: ti-ads8688: Add DT binding documentation

Adding binding documentation for Texas Instruments ADS8688 ADC.
Signed-off-by: NSean Nyekjaer <sean.nyekjaer@prevas.dk>
Reviewed-by: NMartin Hundebøll <martin.hundeboll@prevas.dk>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/ti-ads8688.txt

+* Texas Instruments' ADS8684 and ADS8688 ADC chip
+
+Required properties:
+ - compatible: Should be "ti,ads8684" or "ti,ads8688"
+ - reg: spi chip select number for the device
+
+Recommended properties:
+ - spi-max-frequency: Definition as per
+		Documentation/devicetree/bindings/spi/spi-bus.txt
+
+Optional properties:
+ - vref-supply: The regulator supply for ADC reference voltage
+
+Example:
+adc@0 {
+	compatible = "ti,ads8688";
+	reg = <0>;
+	vref-supply = <&vdd_supply>;
+	spi-max-frequency = <1000000>;
+};