Fencei memory-changing vp for 40x
Created by: silabs-robin
This PR adds a virtual peripheral to 40x for testing fencei, as per the vplan. It listens for a request on the fencei flush interface and then changes the memory for both rtl and iss based on the enable/address/data registers in the vp. The PR includes a directed test and dv random test for using the vp.