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-mobl3Signed-off-by: Dan Williams <dan.j.williams@intel.com>

tools/testing/cxl/test/cxl.c

@@ -68,15 +68,19 @@ static struct acpi_device acpi0017_mock;
 static struct acpi_device host_bridge[NR_BRIDGES] = {
 	[0] = {
 		.handle = &host_bridge[0],
+		.pnp.unique_id = "0",
 	},
 	[1] = {
 		.handle = &host_bridge[1],
+		.pnp.unique_id = "1",
 	},
 	[2] = {
 		.handle = &host_bridge[2],
+		.pnp.unique_id = "2",
 	},
 	[3] = {
 		.handle = &host_bridge[3],
+		.pnp.unique_id = "3",
 	},
 };