Add AES-NI key expansion for 256 bits

include/polarssl/aes.h

@@ -55,6 +55,11 @@ extern "C" {
 
 /**
  * \brief          AES context structure
+ *
+ * \note           buf is able to hold 32 extra bytes, which can be used:
+ *                 - for alignment purposes if VIA padlock is used, and/or
+ *                 - to simplify key expansion in the 256-bit case by
+ *                 generating an extra round key
  */
 typedef struct
 {