of/unittest: breadcrumbs to reduce pain of future maintainers

Fix warnings pointed out by checkpatch.

Checkpatch warns: externs should be avoided in .c files

Reducing pain for future maintainers - adding a comment so that anyone trying
to find where the extern data is created will be able to find it.
(grep will not find that location)

Signed-off-by: Frank Rowand <frank.rowand@sonymobile.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>