Exploit JLink capabilities in AP

Hi,

Alongside @ksigurdsson I've been using pyOCD to test an M3 in a custom ASIC, via a SEGGER JLink debugger.
During this we've noticed that loading and dumping regions of memory is very slow (loadmem of ~10kB took over 5s, which is significant when multiplied by even a handful of test cases).
I traced this to the implementation of _write_memory_block32 in coresight/ap.py, which ends up calling across to pylink (and thus the actual jlink C library) once per 32-bit write.

As an experiment I hacked this function to directly call memory_write in pylink/jlink.py, thus passing the whole block to the jlink API in one go. loadmem now takes ~50ms for 10kB.

It seems to me that given the structure of the pyOCD code, the right way to expliot the abilities of the jlink library is to create a specialised implementation of the AP classes in ap.py. Do you agree? If so, I'm happy to contribute the implementation (with a little guidance on where to put it and how to choose to use the specialised version).

Thanks

Start with _write_memory_block32 in coresight/ap.py and compare it with memory_write in pylink/jlink.py. Trace how AP implementations are selected, then benchmark block memory loading and dumping with a JLink. Done means the JLink path uses block operations and substantially reduces the reported transfer time without changing other AP behavior.