iio: adc: ad7380: move internal reference voltage to chip_info

Move the internal reference voltage value to the chip_info structure.

Before this change, only ADAQ chips could be internal_ref_only and only
non-ADAQ chips could be external_ref_only. Now, this restriction is
removed.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250401-iio-ad7380-add-ad7389-4-v1-3-23d2568aa24f@baylibre.com


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