Wrap AxiTop into an AxiTopSim for simulation

[johanpel]

Currently the AXI top-level output of Fletchgen using --axi doesn't provide a simulation like the simulation top-level that it can generate using the --sim flag.

It would be nice to just wrapception the AXI top into a simulation top level, so the same simulation capabilities of the normal simulation top-level become available to the AXI top-level as well, testing the AXI top-level itself in the meanwhile for all different codegen tests.

This would also enable the possibility of adding a protocol checker on the axi-simulation top level.

Alternatively we could just make the simulation top-level always instantiate the AXI top wrapper.