SPDX: add X11 SPDX-License-Identifier

These is a growing trend to license DT files dual GPL and X11
especially in the Linux community.  It allows easier reuse of
device trees for other software projects.

This commit prepares for doing so in U-Boot too, since DT files are
often copied from the kernel to U-Boot.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Licenses/README

@@ -67,3 +67,4 @@ BSD 3-clause "New" or "Revised" License		BSD-3-Clause	Y		bsd-3-clause.txt	http:/
 IBM PIBS (PowerPC Initialization and		IBM-pibs			ibm-pibs.txt
 	Boot Software) license
 ISC License					ISC		Y		isc.txt			https://spdx.org/licenses/ISC
+X11 License					X11				x11.txt			https://spdx.org/licenses/X11.html

Licenses/x11.txt

@@ -0,0 +1,25 @@
+X11 License
+Copyright (C) 1996 X Consortium
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE X
+CONSORTIUM BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+Except as contained in this notice, the name of the X Consortium shall not be
+used in advertising or otherwise to promote the sale, use or other dealings in
+this Software without prior written authorization from the X Consortium.
+
+X Window System is a trademark of X Consortium, Inc.