cxl/pmem: Export dirty shutdown count via sysfs

cxl/pmem: Export dirty shutdown count via sysfs

Similar to how the acpi_nfit driver exports Optane dirty shutdown count,
introduce:

  /sys/bus/cxl/devices/nvdimm-bridge0/ndbusX/nmemY/cxl/dirty_shutdown

Under the conditions that 1) dirty shutdown can be set, 2) Device GPF
DVSEC exists, and 3) the count itself can be retrieved.

Suggested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://patch.msgid.link/20250220220235.276831-4-dave@stgolabs.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>