dm: core: Switch the testbus driver to use a new struct

At present this driver uses 'priv' struct to hold 'plat' data, which is
confusing. The contents of the strct don't matter, since only dtoc is
using it. Create a new struct with the correct name.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/dm/test.h

@@ -92,6 +92,13 @@ struct dm_test_uclass_priv {
 	int total_add;
 };
 
+/**
+ * struct dm_test_uclass_plat - private plat data for test uclass
+ */
+struct dm_test_uclass_plat {
+	char dummy[32];
+};
+
 /**
  * struct dm_test_parent_data - parent's information on each child
  *