spi: Add RTIO support to SPI

Provides a macro and submit API for SPI drivers to support RTIO.

A copy function enables compatibility with the existing blocking API
and very easily the existing async API as well.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>

drivers/spi/spi_rtio.c

@@ -0,0 +1,11 @@
+/*
+ * Copyright (c) 2023 Intel Corporation
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/drivers/spi.h>
+
+const struct rtio_iodev_api spi_iodev_api = {
+	.submit = spi_iodev_submit,
+};