The System Power Management Controller is a system device defined in
ACPI spec whose purpose is to provide hooks for Operating
System-directed configuration and Power Management (OSPM) to help the
platform achieve higher efficiency. The corollary to this is that the
presence of an SPMC and certain functions makes it a good candidate for
belong to a platform which supports s0ix.
I've had this sitting in my branch for emulated S3 for quite a while. It works
well there and upcoming devices such as NVDIMM may need this before I am ready
to submit my branch for review.