aes: add a define for the size of a block

In the code, we use the size of the key for the
size of the block. It's true when the key is 128 bits,
but it become false for key of 192 bits and 256 bits.
So to prepare the support of aes192  and 256,
we introduce a constant for the iaes block size.

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