spi: axi-spi-engine: implement offload support

Implement SPI offload support for the AXI SPI Engine. Currently, the
hardware only supports triggering offload transfers with a hardware
trigger so attempting to use an offload message in the regular SPI
message queue will fail. Also, only allows streaming rx data to an
external sink, so attempts to use a rx_buf in the offload message will
fail.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
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-v8-7-e48a489be48c@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: dlech

drivers/spi/Kconfig

@@ -179,6 +179,7 @@ config SPI_AU1550
 config SPI_AXI_SPI_ENGINE
 	tristate "Analog Devices AXI SPI Engine controller"
 	depends on HAS_IOMEM
+	select SPI_OFFLOAD
 	help
 	  This enables support for the Analog Devices AXI SPI Engine SPI controller.
 	  It is part of the SPI Engine framework that is used in some Analog Devices