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>

arch/sandbox/dts/overlay0.dts

@@ -1,3 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ *  Overlay test file
+ */
+
 /dts-v1/;
 /plugin/;
 

arch/sandbox/dts/overlay1.dts

@@ -1,3 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ *  Overlay test file
+ */
+
 /dts-v1/;
 /plugin/;
 

arch/sandbox/dts/sandbox.dts

@@ -1,3 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ *  Main sandbox devicetree
+ */
+
 /dts-v1/;
 
 #include <config.h>

arch/sandbox/dts/sandbox.dtsi

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0+
 /*
  * This is the common sandbox device-tree nodes. This is shared between sandbox
  * and sandbox64 builds.

arch/sandbox/dts/sandbox64.dts

@@ -1,3 +1,7 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ *  Main sandbox64 devicetree
+ */
 /dts-v1/;
 
 #include <config.h>

arch/sandbox/dts/test.dts

@@ -1,3 +1,12 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Devicetree file for running sandbox tests
+ *
+ * This includes lots of extra devices used by various tests.
+ *
+ * Note that SPL use the main sandbox.dts file
+ */
+
 /dts-v1/;
 
 #include <dt-bindings/gpio/gpio.h>