[formal] Opentitan FPV using Symbiyosys

*This is a tracking bug. Comment if you have suggestions or updates*

[Symbiyosys](https://github.com/YosysHQ/SymbiYosys) currently support limited functionalities for FPV methodology.

## Yosys
Symbiyosys uses Yosys to synthesize SV. Current list of unsupported Yosys syntax is listed in the link [here](https://github.com/lowRISC/ibex/issues/60).

- [ ] Use util/syn_yosys.sh to convert SV file to Verilog.

## Symbiyosys
Symbiyosys has been adding supports for FPV related syntax.
Here are the list of [supported syntax](https://symbiyosys.readthedocs.io/en/latest/verific.html#supported-sva-property-syntax)

Still exploring:
- [ ] Symbolic variable
- [ ] Code and COI coverages
- [ ] Import a filelist in .sby file