Export the QoS Throttling Group ID from the CXL Fixed Memory Window
Structure (CFMWS) under the root decoder sysfs attributes as qos_class.

CXL rev3.0 9.17.1.3 CXL Fixed Memory Window Structure (CFMWS)

cxl cli will use this id to match with the _DSM retrieved id for a
hot-plugged CXL memory device DPA memory range to make sure that the
DPA range is under the right CFMWS window.
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/169713681699.2205276.14475306324720093079.stgit@djiang5-mobl3Signed-off-by: Dan Williams <dan.j.williams@intel.com>