patman: Add Cover-letter-cc tag to Cc cover letter to people

The cover letter is sent to everyone who is on the Cc list for any of
the patches in the series. Sometimes it is useful to send just the cover
letter to additional people, so that they are aware of the series, but
don't need to wade through all the individual patches.

Add a new Cover-letter-cc tag for this purpose.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>