ofw_cpu: quiet secondary CPU devices

We already do plenty to announce the different CPUs in dmesg. Follow the
ACPI CPU strategy of reporting the first CPU device, but quieting the
rest for non-verbose boot. This cuts down slightly on dmesg output.

Reviewed by: manu, jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40243