board_r: Add missing return to initr_doc

I happened to spot this while working in the area.

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Acked-by: Simon Glass <sjg@chromium.org>
Cc: Simon Glass <sjg@chromium.org>
diff --git a/common/board_r.c b/common/board_r.c
index 8e7a3ac..ba9a68d 100644
--- a/common/board_r.c
+++ b/common/board_r.c
@@ -587,6 +587,7 @@
 {
 	puts("DOC:   ");
 	doc_init();
+	return 0;
 }
 #endif