Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

tools/dts/sabre.dts

@@ -1,13 +1,4 @@
-/*
- * The code contained herein is licensed under the GNU General Public
- * License. You may obtain a copy of the GNU General Public License
- * Version 2 or later at the following locations:
- *
- * http://www.opensource.org/licenses/gpl-license.html
- * http://www.gnu.org/copyleft/gpl.html
- *
- * @TAG(OTHER_GPL2P)
- */
+/* SPDX-License-Identifier: GPL-2.0-or-later */
 
 /dts-v1/;