iio: adc: ad7606: add support for AD760{7,8,9} parts

iio: adc: ad7606: add support for AD760{7,8,9} parts

The AD7607, AD7608 and AD7609 are some older parts of the AD7606 family.
They are hardware-only, meaning that they don't have any registers
accessible via SPI or Parallel interface.
They are more similar to the AD7605-4 part, which is supported by the
'ad7606' driver, and are configurable via GPIOs.

Like the AD7605-4 part, all 3 parts have 2 CONVST (Conversion Start) pins
(CONVST A and CONVST B). But in practice, these should be tied together to
make reading of samples easier via a serial line.

The AD7607 has an 14-bit resolution and AD7608 & AD7609 have an 18-bit
resolution. The main difference between the AD7608 & AD7609 is that the
AD7609 has a larger range (±10V & ±20V) vs the ±5V & ±10V ranges for AD7608.

However, unlike AD7605-4 part, these 3 parts have oversampling which is
configurable (like for the AD7606 in HW-mode) via GPIOs.

Datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/ad7607.pdf
Datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/ad7608.pdf
Datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/ad7609.pdf

Signed-off-by: Alexandru Ardelean <aardelean@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241025095939.271811-6-aardelean@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>