iio: adc: ad_sigma_delta: add SPI offload support

Add SPI offload support to the ad_sigma_delta module.

When the SPI controller has SPI offload capabilities, the module will
now use that for buffered reads instead of the RDY interrupt trigger.

Drivers that use the ad_sigma_delta module will have to opt into this
by setting supports_spi_offload since each driver will likely need
additional changes before SPI offload can be used. This will allow us
to gradually enable SPI offload support for each driver.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250701-iio-adc-ad7173-add-spi-offload-support-v3-11-42abb83e3dac@baylibre.com


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