LICENSES: Add the CC-BY-4.0 license

Add the full text of the CC-BY-4.0 license to the kernel tree as well as
the required tags for reference and tooling.

The license text was copied directly from the following url, but for
clarification a 'Creative Commons' was added before 'Attribution 4.0
International' in the first line:
https://creativecommons.org/licenses/by/4.0/legalcode.txt

CC-BY-4.0 is GPLv2 compatible, but when for example used for the
kernel's documentation it can easily happen that sphinx during
processing combines it with text or code from files using a more
restrictive license[1]. This bears pitfalls, hence point that risk out
and suggest to only use this license in combination with the GPLv2.

[1] https://lkml.kernel.org/r/20201201144314.GA14256@lst.deSigned-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
CC: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/7115b6c20ae3e6db0370fe4002dd586011205e1c.1607063223.git.linux@leemhuis.infoSigned-off-by: Jonathan Corbet <corbet@lwn.net>