doc: iio: ad7380: describe offload support

Add a section to the ad7380 documentation describing how to use the
driver with SPI offloading.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250310-wip-bl-spi-offload-ad7380-v4-2-b184b37b7c72@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: spectrum70

Documentation/iio/ad7380.rst

@@ -178,6 +178,24 @@ Unimplemented features
 - Power down mode
 - CRC indication
 
+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.
 
 Device buffers
 ==============