iio: adc: ad4691: add SPI offload support

iio: adc: ad4691: add SPI offload support

Add SPI offload support to enable DMA-based, CPU-independent data
acquisition using the SPI Engine offload framework.

When an SPI offload is available (devm_spi_offload_get() succeeds),
the driver registers a DMA engine IIO buffer and uses dedicated buffer
setup operations. If no offload is available the existing software
triggered buffer path is used unchanged.

Both CNV Burst Mode and Manual Mode support offload, but use different
trigger mechanisms:

CNV Burst Mode: the SPI Engine is triggered by the ADC's DATA_READY
signal on the GP pin specified by the trigger-source consumer reference
in the device tree (one cell = GP pin number 0-3). For this mode the
driver acts as both an SPI offload consumer (DMA RX stream, message
optimization) and a trigger source provider: it registers the
GP/DATA_READY output via devm_spi_offload_trigger_register() so the
offload framework can match the '#trigger-source-cells' phandle and
automatically fire the SPI Engine DMA transfer at end-of-conversion.

Manual Mode: the SPI Engine is triggered by a periodic trigger at
the configured sampling frequency. The pre-built SPI message uses
the pipelined CNV-on-CS protocol: N+1 16-bit transfers are issued
for N active channels (the first result is discarded as garbage from
the pipeline flush) and the remaining N results are captured by DMA.

All offload transfers use 16-bit frames (bits_per_word=16, len=2).
The SPI Engine assembles received bits into native 16-bit words before
DMA, so offload samples land in CPU-native byte order (IIO_CPU).
Dedicated channel arrays (AD4691_OFFLOAD_CHANNEL) reflect this: they
omit IIO_BE and carry no soft timestamp (DMA delivers data directly to
userspace). The software triggered-buffer path retains its IIO_BE
channels because bits_per_word=8 causes SPI to deliver bytes MSB-first
into memory, making the on-disk layout big-endian. Both paths use
storagebits=16 as transfers are 16 bits wide in both cases.

IIO_BUFFER_DMAENGINE is selected because the offload path uses
devm_iio_dmaengine_buffer_setup_with_handle() to allocate and
attach the DMA RX buffer to the IIO device.

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