license: Update deprecated SPDX tag GPL-2.0 to GPL-2.0-only

The 'GPL-2.0' license identifier has been deprecated since license
list version 3.0 [1] and replaced by the 'GPL-2.0-only' tag [2].

[1] https://spdx.org/licenses/GPL-2.0.html
[2] https://spdx.org/licenses/GPL-2.0-only.html

Mechanical patch running:

  $ sed -i -e s/GPL-2.0/GPL-2.0-only/ \
    $(git grep -l 'SPDX-License-Identifier: GPL-2.0[ $]' \
        | egrep -v '^linux-headers|^include/standard-headers')

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

scripts/kernel-doc

@@ -1,5 +1,5 @@
 #!/usr/bin/env perl
-# SPDX-License-Identifier: GPL-2.0
+# SPDX-License-Identifier: GPL-2.0-only
 
 use warnings;
 use strict;