Not all child devices of the NVDIMM root device represent DIMM
devices which are present in the system. The spec says (ACPI 6.2,
sec 9.20.2):
> For each NVDIMM present or intended to be supported by platform,
> platform firmware also exposes an NVDIMM device ... under the
> NVDIMM root device.
Present NVDIMM devices are found by walking all of the NFIT
table's SPA ranges, then walking the NVDIMM regions mentioned by
those SPA ranges.
A set of NFIT walking helper functions are introduced to avoid the
need to splat the enumeration logic across several disparate
callbacks.