cxl/memdev: Add numa_node attribute

While CXL memory targets will have their own memory target node,
individual memory devices may be affinitized like other PCI devices.
Emit that attribute for memdevs.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/164298428430.3018233.16409089892707993289.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>