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>
diff --git a/tools/patman/series.py b/tools/patman/series.py
index 8ae218d..da734d9 100644
--- a/tools/patman/series.py
+++ b/tools/patman/series.py
@@ -16,7 +16,7 @@
 
 # Series-xxx tags that we understand
 valid_series = ['to', 'cc', 'version', 'changes', 'prefix', 'notes', 'name',
-                'cover_cc', 'process_log', 'links', 'patchwork_url']
+                'cover_cc', 'process_log', 'links', 'patchwork_url', 'postfix']
 
 class Series(dict):
     """Holds information about a patch series, including all tags.
@@ -133,6 +133,7 @@
             print('Cc:\t ', item)
         print('Version: ', self.get('version'))
         print('Prefix:\t ', self.get('prefix'))
+        print('Postfix:\t ', self.get('postfix'))
         if self.cover:
             print('Cover: %d lines' % len(self.cover))
             cover_cc = gitutil.BuildEmailList(self.get('cover_cc', ''))
@@ -322,4 +323,8 @@
         prefix = ''
         if self.get('prefix'):
             prefix = '%s ' % self['prefix']
-        return '%s%sPATCH%s' % (git_prefix, prefix, version)
+
+        postfix = ''
+        if self.get('postfix'):
+           postfix = ' %s' % self['postfix']
+        return '%s%sPATCH%s%s' % (git_prefix, prefix, postfix, version)