dt-bindings: iio: adc: Add common ADCs properties to a separate file

There are several ADC drivers that depend on the same device tree
bindings. Rather than continue to duplicate the properties, this patch
adds a common adc binding document that can be referenced. For beginning,
only two properties are documented.
Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/adc.txt

+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>;
+			...
+		};
+	};