LICENSES: Add CC-BY-SA-4.0 license text

Add the full text of the CC-BY-SA-4.0 license to the kernel tree.  It was
copied directly from:

   https://spdx.org/licenses/CC-BY-SA-4.0.html#licenseTextSigned-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>