Introduce combined modules

[wirthjohannes]

This PR introduces combined modules (and interfaces) for AXI4, AXI4-Lite and AXI3.
With this you no longer need to instantiate two separate modules for RD and WR.

This change is entirely optional and does not break existing usages.

Besides this, the PR also gets rid of the compiler warnings produced by the dummy modules.

[wirthjohannes]

Replaced the GetS interfaces in the combined modules with the more common Get.
This makes the "normal" usage more in-line with the existing, split modules. But it does not allow snooping of values (for the combined modules).

If snooping is something which we want, we need to to figure out a good way to do this without impacting the "normal" usage. (Also until now snooping was implemented only in some of the split modules)