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.dtsi b/arch/sandbox/dts/sandbox.dtsi
index 200fcab..6cf5f14 100644
--- a/arch/sandbox/dts/sandbox.dtsi
+++ b/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.