crypto: caam - abstract ahash request double buffering

caamhash uses double buffering for holding previous/current
and next chunks (data smaller than block size) to be hashed.

Add (inline) functions to abstract this mechanism.

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