iio: ad7303: Updated documentation

Added documentation about the "adi,use-external-reference"
property.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
iio: ad5064: Allow choosing external supply from device tree

Using the boolean property "adi,use-external-reference" it is
now possible to select the external supply from the device tree.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
iio: ad5064: Added devicetree bindings documentation

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>