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/overlay0.dts b/arch/sandbox/dts/overlay0.dts
index 70c6cf7..9e5f389 100644
--- a/arch/sandbox/dts/overlay0.dts
+++ b/arch/sandbox/dts/overlay0.dts
@@ -1,3 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ *  Overlay test file
+ */
+
 /dts-v1/;
 /plugin/;