crypto: caam - refactor ahash shared descriptor generation

Move ahash shared descriptor generation into a single function.
Currently there is no plan to support ahash on any other interface
besides the Job Ring, thus for now the functionality is not exported.

Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>