iio: adc: ad4000: Add support for SPI offload

FPGA HDL projects can include a PWM generator in addition to SPI-Engine.
The PWM IP is used to trigger SPI-Engine offload modules that in turn set
SPI-Engine to execute transfers to poll data from the ADC. That allows data
to be read at the maximum sample rates. Also, it is possible to set a
specific sample rate by setting the proper PWM duty cycle and related state
parameters, thus allowing an adjustable ADC sample rate when a PWM (offload
trigger) is used in combination with SPI-Engine.

Add support for SPI offload.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/386ce043a0e3fc9e8ff71f17aef8de128ce5869e.1743110188.git.marcelo.schmitt@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>