docs: add SPDX tags to new index files

All those new files I added are under GPL v2.0 license.

Add the corresponding SPDX headers to them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/admin-guide/perf/index.rst

@@ -1,3 +1,5 @@
+.. SPDX-License-Identifier: GPL-2.0
+
 ===========================
 Performance monitor support
 ===========================