Add support for external

[PeterCDMcLean]

As is mentioned in the known limitations, PeakRDL-regblock has no support for external registers or regfile. Having support for external is necessary for certain common use cases:

• FIFO read access
• Mapping to block RAM read
• Any other read that must be qualified with a 'read enable' and has arbitrary latency

My suggestion is that the 'out' struct get a scoped 'read enable' signal for any external read. The 'in' struct will contain a 'valid' signal indicating the read is complete and the 'in' data is to be forwarded back to the axi/ahb/cpu bus

[amykyta3]

Yep thats essentially the plan.

For external addrmap and regfile child nodes, the hwif_in/hwif_out structs will contain handshake signals that follow the internal CPUIF protocol described here: https://peakrdl-regblock.readthedocs.io/en/latest/cpuif/internal_protocol.html

External reg and field nodes will have a simpler handshake that allows for arbitrary access latency. Probably pretty similar to the internal protocol, but just without the address bus.

[amykyta3]

Published in v0.13.0