iio: adc: ad4695: Add support for SPI offload

iio: adc: ad4695: Add support for SPI offload

Add support for SPI offload to the ad4695 driver. SPI offload allows
sampling data at the max sample rate (500kSPS or 1MSPS).

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

[1]: http://analogdevicesinc.github.io/hdl/projects/ad469x_fmc/index.html

Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250207-dlech-mainline-spi-engine-offload-2-v8-14-e48a489be48c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>