iio: adc: ad7173: add SPI offload support

iio: adc: ad7173: add SPI offload support

Enable SPI offload support for the AD7173 ADC driver.

The scan_type used for SPI offload is assuming that we are using the
ad411x_ad717x HDL project [1] which always stores data words in 32-bits.

Link: https://analogdevicesinc.github.io/hdl/projects/ad411x_ad717x/index.html
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250710-iio-adc-ad7173-add-spi-offload-support-v4-1-536857c4e043@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>