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>