its use in upcoming code.
This is inspired by something in jhb's NUMA IRQ allocation patchset.
However, the tricky bit here is that the PXM lookup for a node may
fail, requiring a lookup on the parent node. So if it doesn't
exist, don't fail - just go up to the parent. Only error out of the
lookup is the ACPI lookup returns an error.
Sponsored by: Norse Corp, Inc.