libnvdimm/labels: Introduce CXL labels

Now that all of use sites of label data have been converted to nsl_*
helpers, introduce the CXL label format. The ->cxl flag in
nvdimm_drvdata indicates the label format the device expects. A
follow-on patch allows a bus provider to select the label style.

Note that the EFI definition of the labels represents the Linux "claim
class" with a GUID. The CXL definition of the labels stores the same
identifier in UUID byte order.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/163116432405.2460985.5547867384570123403.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>