cxl: documentation: add missing files to cxl driver-api

Add the missing files into cxl driver api and fix the compile warning.

Suggested-by: Dan Williams <dan.j.williams@intel.com>
Suggested-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Yao Xingtao <yaoxt.fnst@fujitsu.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://patch.msgid.link/20240614084755.59503-3-yaoxt.fnst@fujitsu.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>