patman: Make "Reviewed-by" an important tag

Although "Reviewed-by:" is a tag that gerrit adds, it's also a tag
used by upstream.  Stripping it is undesirable.  In fact, we should
treat it as important.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Simon Glass <sjg@chromium.org>