iio: ti-adc128s052: Add DT binding documentation

Adding binding documentation for Texas Instruments' ADC128S052 ADC chip.
Signed-off-by: NAngelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/ti-adc128s052.txt

+* Texas Instruments' ADC128S052 ADC chip
+
+Required properties:
+ - compatible: Should be "ti,adc128s052"
+ - reg: spi chip select number for the device
+ - vref-supply: The regulator supply for ADC reference voltage
+
+Recommended properties:
+ - spi-max-frequency: Definition as per
+		Documentation/devicetree/bindings/spi/spi-bus.txt
+
+Example:
+adc@0 {
+	compatible = "ti,adc128s052";
+	reg = <0>;
+	vref-supply = <&vdd_supply>;
+	spi-max-frequency = <1000000>;
+};