dax/hmem: Parent dax_hmem devices

dax/hmem: Parent dax_hmem devices

For test purposes it is useful to be able to determine which
"hmem_platform" device is hosting a given sub-device.

Register hmem devices underneath "hmem_platform".

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20260327052821.440749-8-dan.j.williams@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>