iio: adc: ad7944: add support for chain mode

iio: adc: ad7944: add support for chain mode

This adds support for the chain mode of the AD7944 ADC. This mode allows
multiple ADCs to be daisy-chained together. Data from all of the ADCs in
is read by reading multiple words from the first ADC in the chain.

Each chip in the chain adds an extra IIO input voltage channel to the
IIO device.

Only the wiring configuration where the SPI controller CS line is
connected to the CNV pin of all of the ADCs in the chain is supported
in this patch.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240425-iio-ad7944-chain-mode-v1-1-9d9220ff21e1@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>