DV: Test performance counters

[imphil]

We have recently reworked the performance counters to be compliant to the RISC-V spec. The documentation is here: https://ibex-core.readthedocs.io/en/latest/performance_counters.html

Are the performance counters already covered by existing DV? Otherwise this is the issue to track progress towards that :-)

estimate 4

[GregAC]

We should consider some testing of performance counters for V3

[GregAC]

We could do this via a directed test that expects specific values for the counters at the end.

[GregAC]

estimate range: 2 - 4