dm: usb: Add a legacy block interface for USB storage

Add a legacy block interface for USB storage.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/usb_storage.c b/common/usb_storage.c
index 9285c95..80bf3db 100644
--- a/common/usb_storage.c
+++ b/common/usb_storage.c
@@ -1555,4 +1555,11 @@
 	.id		= UCLASS_BLK,
 	.ops		= &usb_storage_ops,
 };
+#else
+U_BOOT_LEGACY_BLK(usb) = {
+	.if_typename	= "usb",
+	.if_type	= IF_TYPE_USB,
+	.max_devs	= USB_MAX_STOR_DEV,
+	.desc		= usb_dev_desc,
+};
 #endif