clk_fixed: quiet a warning message

Frequently there are some unused/unspecified fixed clocks present in a
device tree. Each one emits a warning before it fails to attach, which
results in (sometimes many) repeated messages which are not
user-actionable.

Put this warning behind the bootverbose flag.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56204