dt-bindings: iio: document MAX11100 ADC

* Maxim max11100 Analog to Digital Converter (ADC)

Required properties:

  - compatible: Should be "maxim,max11100"

  - reg: the adc unit address

  - vref-supply: phandle to the regulator that provides reference voltage

Optional properties:

  - spi-max-frequency: SPI maximum frequency

Example:

max11100: adc@0 {

        compatible = "maxim,max11100";

        reg = <0>;

        vref-supply = <&adc0_vref>;

        spi-max-frequency = <240000>;

};