iio: adc: ad7380: enable sequencer for single-ended parts

iio: adc: ad7380: enable sequencer for single-ended parts

ad7386/7/8(-4) single-ended parts have a 2:1 mux in front of each ADC.

>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. First inputs
of muxes correspond to the first half of IIO channels (i.e 0-1 or 0-3)
and second inputs correspond to second half (i.e 2-3 or 4-7)

Currently, the driver supports only sampling first half OR second half of
the IIO channels. To enable sampling all channels simultaneously, these
parts have an internal sequencer that automatically cycles through the
mux entries.

When enabled, the maximum throughput is divided by two. Moreover, the ADCs
need additional settling time, so we add an extra CS toggle to correctly
propagate setting, and an additional spi transfer to read the second
half.

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-v2-6-cd63bf05744c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>