Burst support

[mbertuletti]

This PR adds burst support to the variable latency interconnect.

In systems where multiple interconnects are instantiated hierarchically, the arbitration across multiple initiators at the boundary between hierarchies causes a bottleneck in the request-per-cycle injected into the interconnect. The burst functionality solves the problem. We provide:

• A burst_req_grouper module, that groups a parallel valid request in a burst request,
• A burst_cutter module, that issues two bursts when the parallel request crosses the boundary between hierarchies and targets different arbitrators,
• A burst_manager module, that receives a burst request and translates it into a parallel request,
• A burst_rsp_grouper module, that increases the bandwidth of the response channel to send multiple words from a parallel response with the same resp_valid, thus reducing congestion to the arbiters also on the response path.

More details on the burst protocol applied to a system with hierarchical crossbars and wide memory requests initiators are in the following publication:

TCDM Burst Access: Breaking the Bandwidth Barrier in Shared-L1 RVV Clusters Beyond 1000 FPUs

@misc{shen2025tcdmburstaccessbreaking,
      title={TCDM Burst Access: Breaking the Bandwidth Barrier in Shared-L1 RVV Clusters Beyond 1000 FPUs}, 
      author={Diyou Shen and Yichao Zhang and Marco Bertuletti and Luca Benini},
      year={2025},
      eprint={2501.14370},
      archivePrefix={arXiv},
      primaryClass={cs.AR},
      url={https://arxiv.org/abs/2501.14370}, 
}

This paper is available on arXiv:2501.14370 [cs.AR].