sifive_prci: Delay attachment until after clk_fixed

This avoids noisy output from early attempts to attach before clk_fixed
has attached to the parent clocks.

Reviewed by: kp, mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31023

(cherry picked from commit dcbea9a2f465be1786db21523a7f55db3f7ab3dd)