dm: core: Skip adding uclasses with OF_PLATDATA_INST

There is no need to ever add new uclasses since these are set up at build
time. Update the code to return an error if this is attempted.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/core/uclass.c b/drivers/core/uclass.c
index 1a4ea7a..f753a1f 100644
--- a/drivers/core/uclass.c
+++ b/drivers/core/uclass.c
@@ -148,8 +148,11 @@
 
 	*ucp = NULL;
 	uc = uclass_find(id);
-	if (!uc)
+	if (!uc) {
+		if (CONFIG_IS_ENABLED(OF_PLATDATA_INST))
+			return -ENOENT;
 		return uclass_add(id, ucp);
+	}
 	*ucp = uc;
 
 	return 0;