libnvdimm/label: Define CXL region labels

Add a definition of the CXL 2.0 region label format. Note this is done
as a separate patch to make the next patch that adds namespace label
support easier to read.

Reported-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/163116431893.2460985.4003511000574373922.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>