hps: let Litex handle timing checks
hps: let Litex handle timing checks

When Litex invokes Radiant, it always checks that timing was met.

When Litex invokes Oxide, by default it ignores timing checks but can be
told to enforce them with --nextpnr-timingstrict.

Fixes #58, at least for HPS.

google/CFU-Playground,google/CFU-Playground,google/CFU-Playground,google/CFU-Playground