remove Si5324

[sbourdeauducq]

WRPLL on Si549 is finally making some good progress, and it sounds like we are not going to need the Si5324 in the future.
Si5324 will remain supported in ARTIQ for legacy hardware and for the KC705/ZC706 that NIST are still currently using, but we can probably remove it from Kasli and Kasli-SoC at some point to simplify the hardware. It doesn't sound like there is any Si5324 feature that we cannot implement with Si549+WRPLL.

[marmeladapk]

Unless there's a big hardware change (like from 1.1->2.0) it makes more sense to just DNP Si5324.