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/sandbox.dts b/arch/sandbox/dts/sandbox.dts
index a8938a3..127f168 100644
--- a/arch/sandbox/dts/sandbox.dts
+++ b/arch/sandbox/dts/sandbox.dts
@@ -1,3 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ *  Main sandbox devicetree
+ */
+
 /dts-v1/;
 
 #include <config.h>