Use multiclock mode in formal verification

[janschiefer]

Specify we have async signals / resets in the formal verification configuration with SpinalFormalConfig( _hasAsync = true ) .
This works for SpinalHDL 1.10.1.

With the newer SpinalHDL versions ( dev-branch ) we could use FormalConfig.withAsync() .

[Readon]

Hi the withAsync function has been added two years ago. I suppose it could be used after v1.7.0. :)
In addition, the cross-clocking feature is not used as a default setting because it is much more complex than the synchronized clocking feature, as it involves the concept of global clocking.