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/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 751c13b..42ceeb9 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -190,6 +190,12 @@
 		fake-host-hwaddr = [00 00 66 44 22 22];
 	};
 
+	firmware {
+		sandbox_firmware: sandbox-firmware {
+			compatible = "sandbox,firmware";
+		};
+	};
+
 	gpio_a: base-gpios {
 		compatible = "sandbox,gpio";
 		gpio-controller;