misc: Clean up and complete the gem5<->SystemC-TLM bridge [4/10]

The current TLM bridge only provides a Slave Port that allows the gem5
world to send request to the SystemC world. This patch series refractors
and cleans up the existing code, and adds a Master Port that allows the
SystemC world to send requests to the gem5 world.

This patch:
* Move common code of the example to a common directory. Move the cli
* parsing from the SimControl module to a separate example object. Add
* comments describing the Gem5SimControl module.

Testing Done: Examples compile and run.

Reviewed at http://reviews.gem5.org/r/3695/

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>