iio: ad7380: add support for SPI offload

iio: ad7380: add support for SPI offload

Add support for SPI offload to the ad7380 driver. SPI offload allows
sampling data at the max sample rate (2MSPS with one SDO line).

This is developed and tested against the ADI example FPGA design for
this family of ADCs [1].

[1]: http://analogdevicesinc.github.io/hdl/projects/ad738x_fmc/index.html

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250310-wip-bl-spi-offload-ad7380-v4-1-b184b37b7c72@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>