DT: iio: adc: Add CC_10001 binding documentation

* Cosmic Circuits - Analog to Digital Converter (CC-10001-ADC)

Required properties:

  - compatible: Should be "cosmic,10001-adc"

  - reg: Should contain adc registers location and length.

  - clock-names: Should contain "adc".

  - clocks: Should contain a clock specifier for each entry in clock-names

  - vref-supply: The regulator supply ADC reference voltage.

Optional properties:

  - adc-reserved-channels: Bitmask of reserved channels,

    i.e. channels that cannot be used by the OS.

Example:

adc: adc@18101600 {

	compatible = "cosmic,10001-adc";

	reg = <0x18101600 0x24>;

	adc-reserved-channels = <0x2>;

	clocks = <&adc_clk>;

	clock-names = "adc";

	vref-supply = <&reg_1v8>;

};