cxl/debug: Move debugfs init to cxl_core_init()

In preparation for a new cxl debugfs file, move 'cxl' directory
establishment and teardown to the core and let subsequent init routines
reference that setup.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/165603884654.551046.4962104601691723080.stgit@dwillia2-xfh


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