add SGs / CSFB related dot + msc files

Those graphs + message sequence charts are not yet used by any
of our manuals, but they should become used by the OsmoMSC user
manual once SGs interface support is added.

Related: OS#2583
Change-Id: Idfd3a66c18131b5458d183b8e66f62eaaab65991
5 files changed
tree: cde949b53c5b48337f72b5c6f2fceee5b09b1709
  1. doc/