tools/testing/cxl: add mechanism to lock mem device for testing

The mock cxl mem devs needs a way to go into "locked" status to simulate
when the platform is rebooted. Add a sysfs mechanism so the device security
state is set to "locked" and the frozen state bits are cleared.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/166983617602.2734609.7042497620931694717.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>