Documentation: DT: Document rx51-battery binding

Add devicetree binding documentation for rx51-battery,
which is a simple A/D converter consumer.
Signed-off-by: NSebastian Reichel <sre@kernel.org>

Documentation/devicetree/bindings/power/rx51-battery.txt

+Binding for Nokia N900 battery
+
+The Nokia N900 battery status can be read via the TWL4030's A/D converter.
+
+Required properties:
+- compatible: Should contain one of the following:
+ * "nokia,n900-battery"
+- io-channels: Should contain IIO channel specifiers
+               for each element in io-channel-names.
+- io-channel-names: Should contain the following values:
+ * "temp" - The ADC channel for temperature reading
+ * "bsi"  - The ADC channel for battery size identification
+ * "vbat" - The ADC channel to measure the battery voltage
+
+Example from Nokia N900:
+
+battery: n900-battery {
+	compatible = "nokia,n900-battery";
+	io-channels = <&twl4030_madc 0>,
+		      <&twl4030_madc 4>,
+		      <&twl4030_madc 12>;
+	io-channel-names = "temp",
+			   "bsi",
+			   "vbat";
+};