cxl/mem: Move some definitions to mem.h

In preparation for sharing cxl.h with other generic CXL consumers,
move / consolidate some of the memory device specifics to mem.h.

The motivation for moving out of cxl.h is to maintain least privilege
access to memory-device details since cxl.h is used in multiple files.
The motivation for moving definitions into a new mem.h header is for
code readability and organization. I.e. minimize implementation details
when reading data structures and other definitions.

Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/162096970932.1865304.14510894426562947262.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>