patman: Add "postfix" support to patch subjects

In some communities, it may be necessary to append something after PATCH
in the subject line. For example, the Linux networking subsystem
expects [1] patch subject prefixes like [RFC PATCH net-next 0/99]. This
adds support for such "postfix"s to patman. Although entirely cosmetic,
it is still nice to have.

[1] https://www.kernel.org/doc/html/latest/networking/netdev-FAQ.html#how-do-i-indicate-which-tree-net-vs-net-next-my-patch-should-be-in

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

tools/patman/patchstream.py

@@ -596,6 +596,8 @@ class PatchStream:
         # These seem like they would be nice to include.
         if 'prefix' in self.series:
             parts.append(self.series['prefix'])
+        if 'postfix' in self.series:
+            parts.append(self.serties['postfix'])
         if 'version' in self.series:
             parts.append("v%s" % self.series['version'])