cxl: Move cxl feature command structs to user header

In preparation for cxl fwctl enabling, move data structures related to
cxl feature commands to a user header file.

Reviewed-by; Jonathan Cameron <Jonathan.Cameron@huawei.com>

Link: https://patch.msgid.link/r/20250307205648.1021626-3-dave.jiang@intel.com


Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Li Ming <ming.li@zohomail.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>