dt-bindings: iio/adc: Add docs for Ingenic JZ47xx SoCs ADC.

Add documentation for the ADC controller on JZ47xx SoCs,
used by the ingenic-adc driver.
Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/ingenic,adc.txt

+* Ingenic JZ47xx ADC controller IIO bindings
+
+Required properties:
+
+- compatible: Should be one of:
+  * ingenic,jz4725b-adc
+  * ingenic,jz4740-adc
+- reg: ADC controller registers location and length.
+- clocks: phandle to the SoC's ADC clock.
+- clock-names: Must be set to "adc".
+- #io-channel-cells: Must be set to <1> to indicate channels are selected
+  by index.
+
+ADC clients must use the format described in iio-bindings.txt, giving
+a phandle and IIO specifier pair ("io-channels") to the ADC controller.
+
+Example:
+
+#include <dt-bindings/iio/adc/ingenic,adc.h>
+
+adc: adc@10070000 {
+	compatible = "ingenic,jz4740-adc";
+	#io-channel-cells = <1>;
+
+	reg = <0x10070000 0x30>;
+
+	clocks = <&cgu JZ4740_CLK_ADC>;
+	clock-names = "adc";
+
+	interrupt-parent = <&intc>;
+	interrupts = <18>;
+};
+
+adc-keys {
+	...
+	compatible = "adc-keys";
+	io-channels = <&adc INGENIC_ADC_AUX>;
+	io-channel-names = "buttons";
+	...
+};
+
+battery {
+	...
+	compatible = "ingenic,jz4740-battery";
+	io-channels = <&adc INGENIC_ADC_BATTERY>;
+	io-channel-names = "battery";
+	...
+};