misc: Clean up and complete the gem5<->SystemC-TLM bridge [2/10]

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:
* Add the Master Port. Add an example application that isslustrates its
* use.

Testing Done: A simple example application consisting of a TLM traffic
generator and a gem5 memory is part of the patch.

Reviewed at http://reviews.gem5.org/r/3528/

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