srs_enb: add num_cells param to srsENB

this is needed to support the upcoming multi-cell feature in srsENB.
It's required for carrier aggregation (CA) but can also
be used for (currently) two independent 4G cells.

This commit adds baseline support for the parameter. Upcoming
commits will add hooks to configure the cell's EARFCN, etc.

Change-Id: I6d1658bf820163aee03b64b0f4ac68046c243f37
5 files changed