tools/testing/cxl: Add hostbridge UID string for cxl_test mock hb devices

In order to support acpi_device_uid() call, add static string to
acpi_device->pnp.unique_id.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/170319622564.2212653.1534465446670631698.stgit@djiang5-mobl3


Signed-off-by: Dan Williams <dan.j.williams@intel.com>