dm: Cast away the const-ness of the global_data pointer

In a very few cases we need to adjust the driver model root device, such as
when setting it up at initialisation. Add a macro to make this easier.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/core/uclass.c b/drivers/core/uclass.c
index f6867e4..34723ec 100644
--- a/drivers/core/uclass.c
+++ b/drivers/core/uclass.c
@@ -75,7 +75,7 @@
 	uc->uc_drv = uc_drv;
 	INIT_LIST_HEAD(&uc->sibling_node);
 	INIT_LIST_HEAD(&uc->dev_head);
-	list_add(&uc->sibling_node, &gd->uclass_root);
+	list_add(&uc->sibling_node, &DM_UCLASS_ROOT_NON_CONST);
 
 	if (uc_drv->init) {
 		ret = uc_drv->init(uc);