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>
diff --git a/Licenses/README b/Licenses/README
index 9b9a462..731d45c 100644
--- a/Licenses/README
+++ b/Licenses/README
@@ -67,3 +67,4 @@
 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