crypto: stm32 - Move hash state into separate structure

Create a new struct stm32_hash_state so that it may be exported
in future instead of the entire request context.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>