iio: adc: Add driver support for AD7292

The AD7292 is a 10-bit monitor and control system with ADC, DACs,
temperature sensor, and GPIOs.

Configure AD7292 devices in direct access mode, enabling single-ended
ADC readings.

Datasheet:
Link: https://www.analog.com/media/en/technical-documentation/data-sheets/ad7292.pdf

Signed-off-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>