dt-bindings: iio: adc: add driver for the ti-adc084s021 chip

This adds support for the Texas Instruments ADC084S021 ADC chip.
Signed-off-by: Mårten Lindahl <martenli@axis.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/ti-adc084s021.txt

+* Texas Instruments' ADC084S021
+
+Required properties:
+ - compatible        : Must be "ti,adc084s021"
+ - reg               : SPI chip select number for the device
+ - vref-supply       : The regulator supply for ADC reference voltage
+ - spi-cpol          : Per spi-bus bindings
+ - spi-cpha          : Per spi-bus bindings
+ - spi-max-frequency : Per spi-bus bindings
+
+Example:
+adc@0 {
+	compatible = "ti,adc084s021";
+	reg = <0>;
+	vref-supply = <&adc_vref>;
+	spi-cpol;
+	spi-cpha;
+	spi-max-frequency = <16000000>;
+};