LICENSES/GPL2.0: Add GPL-2.0-only/or-later as valid identifiers

Quite some files have been flagged with the new GPL-2.0-only and
GPL-2.0-or-later identifiers which replace the original GPL-2.0 and
GPL-2.0+ identifiers in the SPDX license identifier specification, but the
identifiers are not mentioned as valid in the GPL-2.0 license file.

Add them to the license file and to the Linux-syscall-note exception to
make everything consistent again.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>