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According to Documentation/process/license-rules.rst, move the SPDX
License Identifier to the very top of the file.  I used C++ comment
style not only for the SPDX line but for the entire block because
this seems Linus' preference [1].  I also dropped the parentheses to
follow the examples in that document.

[1] https://lkml.org/lkml/2017/11/25/133

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