board_r: Add missing return to initr_doc

I happened to spot this while working in the area.
Signed-off-by: NIan Campbell <ijc@hellion.org.uk>
Acked-by: NSimon Glass <sjg@chromium.org>
Cc: Simon Glass <sjg@chromium.org>

common/board_r.c

@@ -587,6 +587,7 @@ static int initr_doc(void)
 {
 	puts("DOC:   ");
 	doc_init();
+	return 0;
 }
 #endif