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 = <0>: See the IIO bindings section "IIO consumers".

Example node:

	ads1202: adc@0 {
		compatible = "sd-modulator";
		#io-channel-cells = <0>;
	};
