cxl/port: Store the downstream port's Component Register mappings in struct cxl_dport

Same as for ports, also store the downstream port's Component Register
mappings, use struct cxl_dport for that.

Signed-off-by: Robert Richter <rrichter@amd.com>
Signed-off-by: Terry Bowman <terry.bowman@amd.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20230622205523.85375-16-terry.bowman@amd.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>