iio: adc: ad7949: add vref selection support

Add support for selecting the voltage reference from the devicetree.

This change is required to get valid readings with all three
vref hardware configurations supported by the ADC.

For instance if the ADC isn't provided with an external reference,
the sample request must specify an internal voltage reference to get a
valid reading.

Signed-off-by: Liam Beguin <lvb@xiphos.com>
Link: https://lore.kernel.org/r/20210815213309.2847711-4-liambeguin@gmail.com


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