Add FuseSoC support and Github CI actions

[olofk]

This adds a core description file for the zipcpu core that exposes targets for linting and for building a GDSII using OpenLANE. All targets are also implemented as Github actions so that they get run on every push to the repo.

Quick FuseSoC instructions:

#install FuseSoC pip3 install fusesoc #Create and enter a new workspace mkdir workspace && cd workspace #Register zipcpu as a library in the workspace fusesoc library add zipcpu /path/to/zipcpu #...if repo is available locally or... fusesoc library add zipcpu https://github.com/zipcpu/zipcpu #...to get the upstream repo

#To run lint fusesoc run --target=lint zipcpu::zipcpu #To build with OpenLANE running in a docker container EDALIZE_LAUNCHER=el_docker fusesoc run --target=sky130 zipcpu::zipcpu #List all targets fusesoc core show zipcpu::zipcpu