cxl/pmem: add id attribute to CXL based nvdimm

Add an id group attribute for CXL based nvdimm object. The addition allows
ndctl to display the "unique id" for the nvdimm. The serial number for the
CXL memory device will be used for this id.

[
  {
      "dev":"nmem10",
      "id":"0x4",
      "security":"disabled"
  },
]

The id attribute is needed by the ndctl security key management to setup a
keyblob with a unique file name tied to the mem device.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/166983617029.2734609.8251308562882142281.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>