This adds a core description file for the spm core that exposes targets
for linting and for building a GDSII using OpenLANE.

Quick FuseSoC instructions:

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

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