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-uclass.c b/drivers/firmware/firmware-uclass.c
index 3d33b6d..7fcd7fb 100644
--- a/drivers/firmware/firmware-uclass.c
+++ b/drivers/firmware/firmware-uclass.c
@@ -7,7 +7,7 @@
 UCLASS_DRIVER(firmware) = {
 	.id		= UCLASS_FIRMWARE,
 	.name		= "firmware",
-#if CONFIG_IS_ENABLED(OF_CONTROL)
+#if CONFIG_IS_ENABLED(OF_CONTROL) && !CONFIG_IS_ENABLED(OF_PLATDATA)
 	.post_bind	= dm_scan_fdt_dev,
 #endif
 };