Strive to raise the clock frequency to 24 MHz

[dehanj]

With our new tool (tools/run_pnr.sh) we can find a seed that is more optimal for our design, and hence utilize nextpnr in our favour to try to reach a 24 MHz clock frequency.

We have seen that this is possible with our current design in main, but since our final design is not yet complete it might not be possible. We should, however, still strive and make an effort for it.