cxl/core: Emit modalias for CXL devices

In order to enable libkmod lookups for CXL device objects to their
corresponding module, add 'modalias' to the base attribute of CXL
devices.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Link: https://lore.kernel.org/r/164298424120.3018233.15611905873808708542.stgit@dwillia2-desk3.amr.corp.intel.com


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