dt-bindings: iio: document MAX11100 ADC

Add device tree bindings documentation for Maxim MAX11100 single-channel
ADC
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/max11100.txt

+* 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>;
+};