lib: rsa: Fix const-correctness of rsassa_pss functions

Prior to introduction of modifications in rsassa_pss functions
related to padding verification, doing a pass to update
const-correctness in targeted functions to comply with
coding-rules and avoid const-cast

Signed-off-by: SESA644425 <gioja.hermann@non.se.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/image.h

@@ -1309,7 +1309,7 @@ ll_entry_declare(struct crypto_algo, __name, cryptos)
 struct padding_algo {
 	const char *name;
 	int (*verify)(struct image_sign_info *info,
-		      uint8_t *pad, int pad_len,
+		      const uint8_t *pad, int pad_len,
 		      const uint8_t *hash, int hash_len);
 };