rockchip: dts: rk3399-puma: make the DTS dual-licensed

The RK3399-Q7 (Puma) DTS should (of course) be dual-licensed.
This updates the licensing info in the rk3399-puma.dts.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/arm/dts/rk3399-puma.dts

@@ -1,7 +1,7 @@
 /*
  * (C) Copyright 2017 Theobroma Systems Design und Consulting GmbH
  *
- * SPDX-License-Identifier:     GPL-2.0+
+ * SPDX-License-Identifier:     GPL-2.0+	X11
  */
 
 /dts-v1/;