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/arch/Kconfig b/arch/Kconfig
index 11900b0..9b4bcbf 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -89,6 +89,7 @@
 	imply CMD_SF_TEST
 	imply CRC32_VERIFY
 	imply FAT_WRITE
+	imply FIRMWARE
 	imply HASH_VERIFY
 	imply LZMA
 	imply SCSI