clkdom_dump(): improve output text

If the call to clknode_get_freq() returns an error (unlikely), report
this, rather than printing the error code as the clock frequency.

If the clock has no parent (e.g. a fixed reference clock), print "none"
rather than "(NULL)(-1)". This is a more human-legible presentation of the
same information.

Reviewed by: manu
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D44267