image: Return destination node for add_verify_data() method

It is useful to know where the verification data was written. Update the
API to return this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/u-boot/rsa.h b/include/u-boot/rsa.h
index 2ed2ac7..01b480d 100644
--- a/include/u-boot/rsa.h
+++ b/include/u-boot/rsa.h
@@ -61,8 +61,9 @@
  *
  * @info:	Specifies key and FIT information
  * @keydest:	Destination FDT blob for public key data
- * @return: 0, on success, -ENOSPC if the keydest FDT blob ran out of space,
-		other -ve value on error
+ * @return: node offset within the FDT blob where the data was written on
+ *	success, -ENOSPC if the keydest FDT blob ran out of space, other -ve
+ *	value on other error
 */
 int rsa_add_verify_data(struct image_sign_info *info, void *keydest);