We initially wanted to provide different level of support for the platforms/targets, mainly
to avoid too much maintenance and let each contributor update its contributed platforms and
targets, but it's easier to update all platforms/targets all-together when LiteX evolves or
changes (and that's what has been done on litex-boards since the creation of the repository).
So let just simplify things and avoid this differentiation.
At the start output of the pll is not stabilized, which
caused malfunctions when used for sys clock domain.
Use AsyncResetSynchronizer to start clock domains
on pll locked signal.
