iio: adc: ad7768-1: add support for SPI offload

iio: adc: ad7768-1: add support for SPI offload

The AD7768-1 family supports sampling rates up to 1 MSPS, which exceeds
the capabilities of conventional triggered buffer operations due to SPI
transaction overhead and interrupt latency.

Add SPI offload support to enable hardware-accelerated data acquisition
that bypasses software SPI transactions using continuous data streaming.

Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>