Add CVXIF verifplans to Docs/VerifPlans repositories for cva6

Created by: Gchauvon

Hello,

I have work on a Verification Plan for the CoreV-X-Interface.

It has protocol points related to the 3 mandatory interfaces ( issue, commit and result interface ). It also has functional testcases, not related to the protocol but useful as a starting point to verify the interface's functionalities. Related to the tests we need for the verification, I have proposed a list of custom instructions that would be helpful to create those tests and that covers major parts of the X-interface functionalities.

I think this verification plan, at least parts of it, can be shared between cores that implements the CoreV-X-Interface.