Update deprecated SPDX license identifiers

As specified in LICENSES/GPL-2.0:
- GPL-2.0 -> GPL-2.0-only;
- GPL-2.0+ -> GPL-2.0-or-later.

Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Oleksii Kurochko <oleksii.kurochko@gmail.com> [RISC-V]

CODING_STYLE

@@ -20,7 +20,7 @@ SPDX
 New files should start with a single-line SPDX comment to express the
 license, e.g.:
 
-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: GPL-2.0-only */
 
 See LICENSES/ for a list of licenses and SPDX tags currently used.