LICENSES: add the GFDL-1.2 license

LICENSES: add the GFDL-1.2 license

The documentation of OpenOCD is released under the GNU Free
Documentation License, version 1.2, with embedded some part of
OpenOCD code released under the GNU GPL-2.0-or-later.

Update doc/fdl.texi with latest minor fixes as in
	https://www.gnu.org/licenses/old-licenses/fdl-1.2.texi
Update doc/openocd.texi and move here the license chapter title
Add license file LICENSES/preferred/GFDL-1.2 from
	https://www.gnu.org/licenses/old-licenses/fdl-1.2.txt
and add the required tags for reference and tooling, coping
mostly from the Linux kernel license file in the 'deprecated'
folder.
Add a readme file to link to the existing texinfo copy of the
license.

Change-Id: Ief96e0686257be7a70d4eeec442848bd6494763d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5969
Tested-by: jenkins