ACPI: NUMA: map pxms to low node ids

pxms are mapped to low node ids to maintain generic kernel use of
functions such as pxm_to_node() that are used to determine device
affinity.  Otherwise, there is no pxm-to-node and node-to-pxm matching
rule for x86_64 users of NUMA emulation where a single pxm may be bound
to multiple NUMA nodes.

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Len Brown <len.brown@intel.com>