crypto: caam - refactor encryption descriptors generation

Refactor the generation of the authenc, ablkcipher shared descriptors
and exports the functionality, such that they could be shared
with the upcoming caam/qi (Queue Interface) driver.

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