Support the snitch extension

[AntonLydike]

This will be an effort of multiple PRs, we would need to do:

The main goals of this effort are:

• [ ] Add the Xfrep ISA extension
• [x] Add the Xssr ISA extension

In order to achieve this, we need to do the following steps (at least):

• [ ] Add Zicsr extension to the userspace (currently only implemented for the whole machine mode system CPU thingy)
  • Add a CSR bank to the CPU, I'm not sure if the design on the PrivCPU is a good one tbh.
• [x] Add a "streaming register" bank, a rough version was drafted during the hackathon in the snitch branch
• [ ] Figure out a way to make the CPU extensible in a way so that the Xfrep extension doesn't require a custom CPU class