spi: dt-bindings: axi-spi-engine: add SPI offload properties

spi: dt-bindings: axi-spi-engine: add SPI offload properties

The AXI SPI Engine has support for hardware offloading capabilities.
This includes a connection to a DMA controller for streaming RX or TX
data and a trigger input for starting execution of the SPI message
programmed in the offload. It is designed to support up to 32 offload
instances.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250207-dlech-mainline-spi-engine-offload-2-v8-6-e48a489be48c@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>