Documentation: DT: Document twl4030-madc binding

Add devicetree binding documentation for twl4030-madc
analog digital converter.
Signed-off-by: Sebastian Reichel <sre@debian.org>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/iio/adc/twl4030-madc.txt

+* TWL4030 Monitoring Analog to Digital Converter (MADC)
+
+The MADC subsystem in the TWL4030 consists of a 10-bit ADC
+combined with a 16-input analog multiplexer.
+
+Required properties:
+  - compatible: Should contain "ti,twl4030-madc".
+  - interrupts: IRQ line for the MADC submodule.
+  - #io-channel-cells: Should be set to <1>.
+
+Optional properties:
+  - ti,system-uses-second-madc-irq: boolean, set if the second madc irq register
+				    should be used, which is intended to be used
+				    by Co-Processors (e.g. a modem).
+
+Example:
+
+&twl {
+	madc {
+		compatible = "ti,twl4030-madc";
+		interrupts = <3>;
+		#io-channel-cells = <1>;
+	};
+};