Documentation: iio: ad4000: Describe offload support

When SPI offloading is supported, the IIO device provides different sysfs
interfaces to allow using the adjusting the sample rate. Document SPI
offload support for AD4000 and similar devices.

Reviewed-by: David Lechner <[e-mail]>
Signed-off-by: Marcelo Schmitt <[e-mail]>
Link: https://patch.msgid.link/eb94013[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/iio/ad4000.rst

···
 +-------------------------------------------+------------------------------------------+
 | ``in_voltage0-voltage1_scale_available``  | Toggles input span compression           |
 +-------------------------------------------+------------------------------------------+
+
+SPI offload support
+-------------------
+
+To be able to achieve the maximum sample rate, the driver can be used with SPI
+offload engines such as the one usually present in `AXI SPI Engine`_, to provide
+SPI offload support.
+
+.. _AXI SPI Engine: http://analogdevicesinc.github.io/hdl/projects/pulsar_adc/index.html
+
+To keep up with SPI offloading transfer speeds, the ADC must be connected either
+in 3-wire turbo mode or in 3-wire without busy indicator mode and have SPI
+controller CS line connected to the CNV pin.
+
+When set for SPI offload support, the IIO device will provide different
+interfaces.
+
+* Either ``in_voltage0_sampling_frequency`` or
+  ``in_voltage0-voltage1_sampling_frequency`` file is provided to allow setting
+  the sample rate.
+* IIO trigger device is not provided (no ``trigger`` directory).
+* ``timestamp`` channel is not provided.
+
+Also, because the ADC output has a one sample latency (delay) when the device is
+wired in "3-wire" mode and only one transfer per sample is done when using SPI
+offloading, the first data sample in the buffer is not valid because it contains
+the output of an earlier conversion result.