clk_fixed: call clkdom_dump() for verbose boot

This is standard practice for clock drivers that register clocks
dynamically. Nothing else uses the CLK_DEBUG macro.

The result is that the name and frequency of the fixed clock is printed
for a verbose boot, which may aid in debugging.

Reviewed by: manu
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D44265