dt-bindings: iio: adc: ad7949: update voltage reference bindings

Update bindings to describe support for buffered and unbuffered external
voltage references selection, and add adi,internal-ref-microvolt for
internal voltage reference selection.

Signed-off-by: Liam Beguin <lvb@xiphos.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210815213309.2847711-5-liambeguin@gmail.com


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