sysmobts: Document the values of the clock-source in the vty command
sysmobts: Document the values of the clock-source in the vty command

We need to have a documentation for each possible value, add some simple
ones for the available clock sources.

telenoobie/osmo-bts,shimaore/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts,shimaore/osmo-bts,geosphere/osmo-bts,telenoobie/osmo-bts,telenoobie/osmo-bts,geosphere/osmo-bts,telenoobie/osmo-bts,osmocom/osmo-bts,geosphere/osmo-bts,shimaore/osmo-bts,geosphere/osmo-bts,shimaore/osmo-bts