iio: adc: add helpers for parsing ADC nodes

There are ADC ICs which may have some of the AIN pins usable for other
functions. These ICs may have some of the AIN pins wired so that they
should not be used for ADC.

A common way of marking pins that can be used as ADC inputs is to add
corresponding channel@N nodes in the device tree as described in the ADC
	      binding yaml.

Add couple of helper functions which can be used to retrieve the channel
information from the device node.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://patch.msgid.link/f1d8b3e15237947738912c0d297b3e1e21d8b03e.1742560649.git.mazziesaccount@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>