The PRCI module exports three PLLs. Currently only the coreclk/corepll
is registered, so add the logic to register the DDR (memory) and GEMGX
(ethernet) clocks as well.
See also: Chapter 7 of the [[ https://sifive.cdn.prismic.io/sifive%2F834354f0-08e6-423c-bf1f-0cb58ef14061_fu540-c000-v1.0.pdf | FU540 manual ]].