arm64: dts: uniphier: use SPDX-License-Identifier

Follow the recent trend for the license description, and fix the wrongly
stated X11 to MIT.

The X11 license text [1] is explicitly for the X Consortium and has a
couple of extra clauses.  The MIT license text [2] is actually what the
current DT files claim.

[1] https://spdx.org/licenses/X11.html
[2] https://spdx.org/licenses/MIT.html

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