There is an old forum post for the fe310 that might be relevant.
https://forums.sifive.com/t/something-i-learned-about-the-cpu-clock/2635
It points to some freedom-e-sdk code at the end. Here is the source code for that.
https://github.com/sifive/freedom-metal/blob/master/src/drivers/sifive_fe310-g000_pll.c#L344
This is a very different SoC, but it might be the same PLL subsystem so this might be relevant.