rust: bindings: turn SPDX tags into comments

rust: bindings: turn SPDX tags into comments

In markdown `# Foo` generates a top-level heading. This leads to to some
weird, huge SPDX tags being rendered before the start of the actual
content.

Lacking good examples, I just took the syntax that `reuse addheader`[1]
outputs.

[1] https://github.com/fsfe/reuse-tool

Signed-off-by: Erik Schilling <erik.schilling@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>