iio: adc: ad4691: add triggered buffer support

Add buffered capture support using the IIO triggered buffer framework.

CNV Burst Mode: the GP pin identified by interrupt-names in the device
tree is configured as DATA_READY output. The IRQ handler stops
conversions and fires the IIO trigger; the trigger handler executes a
pre-built SPI message that reads all active channels from the AVG_IN
accumulator registers and then resets accumulator state and restarts
conversions for the next cycle.

Manual Mode: CNV is tied to SPI CS so each transfer simultaneously
reads the previous result and starts the next conversion (pipelined
N+1 scheme). At preenable time a pre-built, optimised SPI message of
N+1 transfers is constructed (N channel reads plus one NOOP to drain
the pipeline). The trigger handler executes the message in a single
spi_sync() call and collects the results. An external trigger (e.g.
iio-trig-hrtimer) is required to drive the trigger at the desired
sample rate.

Both modes share the same trigger handler and push a complete scan —
one big-endian 16-bit (__be16) slot per active channel, densely packed
in scan_index order, followed by a timestamp.

The CNV Burst Mode sampling frequency (PWM period) is exposed as a
buffer-level attribute via IIO_DEVICE_ATTR.

Signed-off-by: Radu Sabau <radu.sabau@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

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