powerpc: Fix the NUMA domain list on powernv

Summary:
Consolidate the NUMA associativity handling into a platform function.
Non-NUMA platforms will just fall back to the default (0). Currently
only implemented for powernv, which uses a lookup table to map the
device tree associativity into a system NUMA domain.

Fixes hangs on powernv after r356534, and corrects a fairly longstanding
bug in powernv's NUMA handling, which ended up using domains 1 and 2 for
devices and memory on power9, while CPUs were bound to domains 0 and 1.

Reviewed by: bdragon, luporl
Differential Revision: https://reviews.freebsd.org/D23220