prci: register the DDR and GEMGX PLLs

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. These clocks are unused at the moment.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D23404