[PPC64] Fix NUMA on POWER8

On some POWER8 machines, 'ibm,associativity' property may have 6
cells, which would overflow the 5 cells buffer being used.
There was also an issue with the "check if node is root" part,
that have been fixed too.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D23414