patman: Allow linking a series with patchwork

Add a new Series-links tag to tell patman how to find the series in
patchwork. Each item is the series ID optionally preceded by the series
version that the link refers to. An empty version indicates this is the
latest series.

For example:

   Series-links: 209816 1:203302

Documentation is added in a later patch.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/patman/func_test.py

@@ -417,6 +417,7 @@ Series for my board
 This series implements support
 for my glorious board.
 END
+Series-links: 183237
 ''', 'serial.c', '''The code for the
 serial driver is here''')
         self.make_commit_with_file('bootm: Make it boot', '''