cxl/memdev: Formalize endpoint port linkage

Move the endpoint port that the cxl_mem driver establishes from drvdata
to a first class attribute. This is in preparation for device-memory
drivers reusing the CXL core for memory region management. Those drivers
need a type-safe method to retrieve their CXL port linkage. Leave
drvdata for private usage of the cxl_mem driver not external consumers
of a 'struct cxl_memdev' object.

Reviewed-by: Fan Ni <fan.ni@samsung.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/168679264292.3436160.3901392135863405807.stgit@dwillia2-xfh.jf.intel.com


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