rsa: sig: fix config signature check for fit with padding

The signature check on config node is broken on fit with padding.
To compute the signature for config node, U-Boot compute the
signature on all properties of requested node for this config,
except for the property "data". But, when padding is used for
binary in a fit, there isn't a property "data" but two properties:
"data-offset" and "data-size". So to fix the check of signature,
we also don't use the properties "data-offset" and "data-size"
when checking the signature on config node.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>