iio: adc: ad7380: prepare driver for single-ended parts support

iio: adc: ad7380: prepare driver for single-ended parts support

ad738x family contains single-ended parts that have a 2:1 mux in front
of ADC, so the number of IIO channels is different from the number of
simultaneous channels that can be sampled.

To prepare the support for single-ended parts, introduce a new
num_simultaneous_channels variable. For currently supported parts,
num_simultaneous_channels is equal to num_channels minus 1 (the
timestamps channel)

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-v2-4-cd63bf05744c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>