dm: test: Add "/firmware" node scan test

Add a test which verifies that all subnodes under "/firmware"
nodes are scanned.

Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Added 'imply FIRMWARE' to sandbox Kconfig to fix test failures, fixed
ordering of lines in arch/sandbox/dts/test.dts and test/dm/Makefile,
updated #if condition in drivers/firmware/firmware-uclass.c:
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/firmware/firmware-sandbox.c b/drivers/firmware/firmware-sandbox.c
new file mode 100644
index 0000000..d970d75
--- /dev/null
+++ b/drivers/firmware/firmware-sandbox.c
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * sandbox firmware driver
+ *
+ * Copyright (C) 2018 Xilinx, Inc.
+ */
+
+#include <common.h>
+#include <dm.h>
+
+static const struct udevice_id generic_sandbox_firmware_ids[] = {
+	{ .compatible = "sandbox,firmware" },
+	{ }
+};
+
+U_BOOT_DRIVER(sandbox_firmware) = {
+	.name = "sandbox_firmware",
+	.id = UCLASS_FIRMWARE,
+	.of_match = generic_sandbox_firmware_ids,
+};