Generic resistive touchscreen ADC

Required properties:

 - compatible: must be "resistive-adc-touch"
The device must be connected to an ADC device that provides channels for
position measurement and optional pressure.
Refer to ../iio/iio-bindings.txt for details
 - iio-channels: must have at least two channels connected to an ADC device.
These should correspond to the channels exposed by the ADC device and should
have the right index as the ADC device registers them. These channels
represent the relative position on the "x" and "y" axes.
 - iio-channel-names: must have all the channels' names. Mandatory channels
are "x" and "y".

Optional properties:
 - iio-channels: The third channel named "pressure" is optional and can be
used if the ADC device also measures pressure besides position.
If this channel is missing, pressure will be ignored and the touchscreen
will only report position.
 - iio-channel-names: optional channel named "pressure".

Example:

	resistive_touch: resistive_touch {
		compatible = "resistive-adc-touch";
		touchscreen-min-pressure = <50000>;
		io-channels = <&adc 24>, <&adc 25>, <&adc 26>;
		io-channel-names = "x", "y", "pressure";
	};