misc: Clean up and complete the gem5<->SystemC-TLM bridge [3/10]

misc: Clean up and complete the gem5<->SystemC-TLM bridge [3/10]

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:
 * Simplify the Slave Port by using a simple_initiator_socket.

Testing Done: Example applications are still running.

Reviewed at http://reviews.gem5.org/r/3686/

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