crypto: create generic version of ablk_helper

Create a generic version of ablk_helper so it can be reused
by other architectures.

Change-Id: I0c1931c64c9c09b175a2c72b4bc07ec49ad8d6a4
Acked-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>