cxl: Introduce module_cxl_driver

Many CXL drivers simply want to register and unregister themselves.
module_driver already supported this. A simple wrapper around that
reduces a decent amount of boilerplate in upcoming patches.

Suggested-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/164298415591.3018233.13608495220547681412.stgit@dwillia2-desk3.amr.corp.intel.com


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