sandbox: Add license headers to the dts files

Many of these files are missing a header. Fix this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/sandbox64.dts b/arch/sandbox/dts/sandbox64.dts
index a39f94f..ec53106 100644
--- a/arch/sandbox/dts/sandbox64.dts
+++ b/arch/sandbox/dts/sandbox64.dts
@@ -1,3 +1,7 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ *  Main sandbox64 devicetree
+ */
 /dts-v1/;
 
 #include <config.h>