BSC,BTS: add diagrams of PCU-BTS-NITB-SGSN relationships
BSC,BTS: add diagrams of PCU-BTS-NITB-SGSN relationships

Change-Id: I0eb09706efb768fa4f6810872fb6568cbc9838cb

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts