cxl/core: Track port depth

In preparation for proving CXL subsystem usage of the device_lock()
order track the depth of ports with the expectation that  shallower port
locks can be held over deeper port locks.

Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/164298419321.3018233.4469731547378993606.stgit@dwillia2-desk3.amr.corp.intel.com


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