iio: adc: ad_sigma_delta: Add sequencer support

Some sigma-delta chips support sampling of multiple
channels in continuous mode.

When the operating with more than one channel enabled,
the channel sequencer cycles through the enabled channels
in sequential order, from first channel to the last one.
If a channel is disabled, it is skipped by the sequencer.

If more than one channel is used in continuous mode,
instruct the device to append the status to the SPI transfer
(1 extra byte) every time we receive a sample.
All sigma-delta chips possessing a sampling sequencer have
this ability. Inside the status register there will be
the number of the converted channel. In this way, even
if the CPU won't keep up with the sampling rate, it won't
send to userspace wrong channel samples.

When multiple channels are enabled in continuous mode,
the device needs to perform a measurement on all slots
before we can push to userspace the sample.

If, during sequencing and data reading, a channel measurement
is lost, a desync occurred. In this case, ad_sigma_delta drops
the incomplete sample and waits for the device to send the
measurement on the first active slot.

Co-developed-by: Alexandru Tachici <alexandru.tachici@analog.com>
Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20220322105029.86389-5-alexandru.tachici@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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