Improve the code (safety and readability) by indicating that data passed
through pointer is not modified.  This adds const keyword in many places,
most notably:
 - the driver data (pointer to struct samsung_aes_variant),
 - scatterlist addresses written as value to device registers,
 - key and IV arrays.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>