IIO: Add DT bindings for sigma delta adc modulator

Add documentation of device tree bindings to support
sigma delta modulator in IIO framework.
Signed-off-by: NArnaud Pouliquen <arnaud.pouliquen@st.com>
Acked-by: NRob Herring <robh@kernel.org>
Acked-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/iio/adc/sigma-delta-modulator.txt

+Device-Tree bindings for sigma delta modulator
+
+Required properties:
+- compatible: should be "ads1201", "sd-modulator". "sd-modulator" can be use
+	as a generic SD modulator if modulator not specified in compatible list.
+- #io-channel-cells = <1>: See the IIO bindings section "IIO consumers".
+
+Example node:
+
+	ads1202: adc@0 {
+		compatible = "sd-modulator";
+		#io-channel-cells = <1>;
+	};