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";
};