dt-bindings: power: supply: Add docs for Ingenic JZ47xx SoCs battery.

Add documentation for the ingenic-battery driver, used on JZ47xx SoCs.
Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Documentation/devicetree/bindings/power/supply/ingenic,battery.txt

+* Ingenic JZ47xx battery bindings
+
+Required properties:
+
+- compatible: Must be "ingenic,jz4740-battery".
+- io-channels: phandle and IIO specifier pair to the IIO device.
+  Format described in iio-bindings.txt.
+- monitored-battery: phandle to a "simple-battery" compatible node.
+
+The "monitored-battery" property must be a phandle to a node using the format
+described in battery.txt, with the following properties being required:
+
+- voltage-min-design-microvolt: Drained battery voltage.
+- voltage-max-design-microvolt: Fully charged battery voltage.
+
+Example:
+
+#include <dt-bindings/iio/adc/ingenic,adc.h>
+
+simple_battery: battery {
+	compatible = "simple-battery";
+	voltage-min-design-microvolt = <3600000>;
+	voltage-max-design-microvolt = <4200000>;
+};
+
+ingenic_battery {
+	compatible = "ingenic,jz4740-battery";
+	io-channels = <&adc INGENIC_ADC_BATTERY>;
+	io-channel-names = "battery";
+	monitored-battery = <&simple_battery>;
+};