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>
diff --git a/tools/dtoc/test_dtoc.py b/tools/dtoc/test_dtoc.py
index c81bcc9..8bac207 100755
--- a/tools/dtoc/test_dtoc.py
+++ b/tools/dtoc/test_dtoc.py
@@ -616,7 +616,7 @@
 u8 _denx_u_boot_test_bus_priv_some_bus[sizeof(struct dm_test_priv)]
 \t__attribute__ ((section (".priv_data")));
 #include <dm/test.h>
-u8 _denx_u_boot_test_bus_ucplat_some_bus[sizeof(struct dm_test_uclass_priv)]
+u8 _denx_u_boot_test_bus_ucplat_some_bus[sizeof(struct dm_test_uclass_plat)]
 \t__attribute__ ((section (".priv_data")));
 #include <test.h>