Check intended semantics of rvfi_mode

Created by: Silabs-ArjanB

https://github.com/openhwgroup/cv32e40x/pull/213 was merged before we had clarity on the supposed meaning of rvfi_mode (whether mprv needs to factor into this signal or not).

The current implementation is not aligned with Ibex.

This needs to be checked and if needed the RVFI + documentation needs to be adapted accordingly.