MFC: r258722, r258757

MFC:	r258722, r258757

r258722:
Give some output about the CPU clock on IBMPOWER machines, currently read
from OF. Linux does it similar, means they also read the OF values and
display them.
r258757:
Use the Open Firmware-based CPU frequency determination as a generic
fallback if we can't measure CPU frequency. This is also useful on a
variety of embedded systems using FDT.

git-svn-id: svn://svn.freebsd.org/base/stable/10@259256 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f