I have booted this with the qemu "sifive_u" platform, and dmesg shows
that the clocks are registered. I would prefer this set of changes be
tested on a real SiFive Unleashed though.
Sysctl output showing the new PLLs:
```
dev.fu540prci.0.clocks: coreclk ddrclk gemgxclk tlclk
...
hw.clock.gemgxclk.enable_cnt: 0
hw.clock.gemgxclk.childrens:
hw.clock.gemgxclk.parents: hfclk rtcclk
hw.clock.gemgxclk.parent: hfclk
hw.clock.gemgxclk.frequency: 0
hw.clock.ddrclk.enable_cnt: 0
hw.clock.ddrclk.childrens:
hw.clock.ddrclk.parents: hfclk rtcclk
hw.clock.ddrclk.parent: hfclk
hw.clock.ddrclk.frequency: 0
```