iio: adc: ad7380: add support for single-ended parts

Adding ad7386/7/8 (16/14/12 bits) unsigned, dual simultaneous sampling,
single-ended compatible parts, and the corresponding ad7386-4/7-4/8-4
4 channels.

These parts have a 2:1 multiplexer in front of each ADC. They also include
additional configuration registers that allow for either manual selection
or automatic switching (sequencer mode), of the multiplexer inputs.
This commit focus on integrating manual selection. Sequencer mode will
be implemented later.

>From an IIO point of view, all inputs are exported, i.e ad7386/7/8
export 4 channels and ad7386-4/7-4/8-4 export 8 channels.

Inputs AinX0 of multiplexers correspond to the first half of IIO channels
(i.e 0-1 or 0-3) and inputs AinX1 correspond to second half (i.e 2-3 or
4-7). Example for AD7386/7/8 (2 channels parts):

          IIO   | AD7386/7/8
                |         +----------------------------
                |         |     _____        ______
                |         |    |     |      |      |
       voltage0 | AinA0 --|--->|     |      |      |
                |         |    | mux |----->| ADCA |---
       voltage2 | AinA1 --|--->|     |      |      |
                |         |    |_____|      |_____ |
                |         |     _____        ______
                |         |    |     |      |      |
       voltage1 | AinB0 --|--->|     |      |      |
                |         |    | mux |----->| ADCB |---
       voltage3 | AinB1 --|--->|     |      |      |
                |         |    |_____|      |______|
                |         |
                |         +----------------------------

When switching channel, the ADC require an additional settling time.
According to the datasheet, data is valid on the third CS low. We already
have an extra toggle before each read (either direct reads or buffered
reads) to sample correct data, so we just add a single CS toggle at the
end of the register write.

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240731-ad7380-add-single-ended-chips-v2-5-cd63bf05744c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: justeph