drivers: spi: add dummy driver for vnd,spi

We will need this to be able to DEVICE_DT_GET() bus devices from
tests/drivers/build_all in an upcoming commit.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

drivers/spi/spi_test.c

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2021, Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/*
+ * This is not a real SPI driver. It is used to instantiate struct
+ * devices for the "vnd,spi" devicetree compatible used in test code.
+ */
+
+#include <zephyr.h>
+#include <drivers/spi.h>
+
+#define DT_DRV_COMPAT vnd_spi
+
+static int vnd_spi_transceive(const struct device *dev,
+			      const struct spi_config *spi_cfg,
+			      const struct spi_buf_set *tx_bufs,
+			      const struct spi_buf_set *rx_bufs)
+{
+	return -ENOTSUP;
+}
+
+#ifdef CONFIG_SPI_ASYNC
+static int vnd_spi_transceive_async(const struct device *dev,
+				    const struct spi_config *spi_cfg,
+				    const struct spi_buf_set *tx_bufs,
+				    const struct spi_buf_set *rx_bufs,
+				    struct k_poll_signal *async)
+{
+	return -ENOTSUP;
+}
+#endif
+
+static int vnd_spi_release(const struct device *dev,
+			   const struct spi_config *spi_cfg)
+{
+	return -ENOTSUP;
+}
+
+static const struct spi_driver_api vnd_spi_api = {
+	.transceive = vnd_spi_transceive,
+#ifdef CONFIG_SPI_ASYNC
+	.transceive_async = vnd_spi_transceive_async,
+#endif
+	.release = vnd_spi_release,
+};
+
+static int vnd_spi_init(const struct device *dev)
+{
+	return 0;
+}
+
+#define VND_SPI_INIT(n)						\
+	DEVICE_DT_INST_DEFINE(n, &vnd_spi_init, device_pm_control_nop,	\
+			      NULL, NULL, POST_KERNEL,			\
+			      CONFIG_KERNEL_INIT_PRIORITY_DEVICE,	\
+			      &vnd_spi_api);
+
+DT_INST_FOREACH_STATUS_OKAY(VND_SPI_INIT)