Reference model

Created by: torjeikenes

This draft PR brings in some of my work implementing a reference model with the goal of verifying asynchronous events. The RM contains a core-specific pipeline shell, built around a core-independent ISS. The ISS is currently Spike from vendor/riscv/, with modifications done on another branch from master. This PR is therefore not currently functional and depends on #2427 before both branches can be merged. As discussed with @silabs-robin, I am submitting this PS as a draft in order to get feedback and track what changes need to be made.