iio: adc: ad799x: add reference voltage capability to chip_info

iio: adc: ad799x: add reference voltage capability to chip_info

If the chip supports an external reference voltage
on REFIN pin then the "vref-supply" regulator may be used.

This commit partially refactors 6b104e7895ab16b9b7f466c5f2ca282b87f661e8
to add the capability of the chip to have an external
voltage reference and then remove the ugly conditional check
on chip id.

Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Stefano Manni <stefano.manni@gmail.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250807074850.130831-2-stefano.manni@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>