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

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

The AD7380 family has some compatible single-ended chips: AD7386/7/8(-4).
These single-ended chips 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. Add a section to describe this.

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-v2-7-cd63bf05744c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>