nvdimm, btt: add a size attribute for BTTs

To be consistent with other namespaces, expose a 'size' attribute for
BTT devices also.

Cc: Dan Williams <dan.j.williams@intel.com>
Reported-by: Linda Knippers <linda.knippers@hpe.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>