Common ADCs properties

Optional properties for child nodes:
- bipolar : Boolean, if set the channel is used in bipolar mode.
- diff-channels : Differential channels muxed for this ADC. The first value
		specifies the positive input pin, the second value the negative
		input pin.

Example:
	adc@0 {
		compatible = "some,adc";
		...
		channel@0 {
			bipolar;
			diff-channels = <0 1>;
			...
		};

		channel@1 {
			diff-channels = <2 3>;
			...
		};
	};