doc: iio: ad7944: describe offload support

Add a section to the ad7944 documentation describing how to use the
driver with SPI offloading.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250207-dlech-mainline-spi-engine-offload-2-v8-12-e48a489be48c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: dlech

Documentation/iio/ad7944.rst

@@ -46,6 +46,8 @@ CS mode, 3-wire, without busy indicator
 To select this mode in the device tree, set the ``adi,spi-mode`` property to
 ``"single"`` and omit the ``cnv-gpios`` property.
 
+This is the only wiring configuration supported when using `SPI offload support`_.
+
 CS mode, 4-wire, without busy indicator
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -106,7 +108,6 @@ Unimplemented features
 ----------------------
 
 - ``BUSY`` indication
-- ``TURBO`` mode
 
 
 Device attributes
@@ -147,6 +148,27 @@ AD7986 is a fully-differential ADC and has the following attributes:
 In "chain" mode, additional chips will appear as additional voltage input
 channels, e.g. ``in_voltage2-voltage3_raw``.
 
+SPI offload support
+===================
+
+To be able to achieve the maximum sample rate, the driver can be used with the
+`AXI SPI Engine`_ to provide SPI offload support.
+
+.. _AXI SPI Engine: http://analogdevicesinc.github.io/hdl/projects/pulsar_adc/index.html
+
+When SPI offload is being used, some attributes will be different.
+
+* ``trigger`` directory is removed.
+* ``in_voltage0_sampling_frequency`` attribute is added for setting the sample
+  rate.
+* ``in_voltage0_sampling_frequency_available`` attribute is added for querying
+  the max sample rate.
+* ``timestamp`` channel is removed.
+* Buffer data format may be different compared to when offload is not used,
+  e.g. the ``in_voltage0_type`` attribute.
+
+If the ``turbo-gpios`` property is present in the device tree, the driver will
+turn on TURBO during buffered reads and turn it off otherwise.
 
 Device buffers
 ==============