AHB Dummy Subordinate

[rishyak]

Implement a dummy subordinate with all features required by a subordinate to provide an outline for what it needs to do on the bus.

This includes:

• Data read/write
• Responses, including two-cycle responses
• Burst transfers
• Protected transfers

[rishyak]

@kevincyap I believe this is done? Can we PR and merge whilst you work on the memory controller?