iio: adc: ad4695: Add driver for AD4695 and similar ADCs

iio: adc: ad4695: Add driver for AD4695 and similar ADCs

This is a new driver for Analog Devices Inc. AD4695 and similar ADCs.
The initial driver supports initializing the chip including configuring
all possible LDO and reference voltage sources as well as any possible
voltage input channel wiring configuration.

Only the 4-wire SPI wiring mode where the CNV pin is tied to the CS pin
is supported at this time. And reading sample data from the ADC can only
be done in direct mode for now.

Co-developed-by: Ramona Gradinariu <ramona.gradinariu@analog.com>
Signed-off-by: Ramona Gradinariu <ramona.gradinariu@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240711-iio-adc-ad4695-v4-2-c31621113b57@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>