cxl/core/region: move dax region device logic into region_dax.c

cxl/core/region: move dax region device logic into region_dax.c

core/region.c is overloaded with per-region control logic (pmem, dax,
sysram, etc). Move the CXL DAX region device infrastructure from
region.c into a new region_dax.c file.

This will also allow us to add additional dax-driver integration paths
that don't further dirty the core region.c logic.

No functional changes.

Signed-off-by: Gregory Price <gourry@gourry.net>
Co-developed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/20260327020203.876122-3-gourry@gourry.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>