PPC64: Get the timestap from the proper OF field

PPC64: Get the timestap from the proper OF field

Summary:
After revision rS328534('PPC64: use hwref instead of cpuid'), FreeBSD on
powerpc64 virtual machine panics since it is unable to read the
timebase, showing the following error:

     get-property for timebase-frequency on zero phandle

     panic: Unable to determine timebase frequency!

With the change above,  cpuref->cr_hwref does not contain the phandle
anymore, thus, it never reads the proper CPU entry in OF.

Submitted by:	Breno Leitao
Differential Revision:	https://reviews.freebsd.org/D14204