crypto: doc - add skcipher API documentation

The crypto API received the skcipher API which is intended to replace
the ablkcipher and blkcipher API. This patch adds the skcipher API
documentation to the DocBook, updates the code sample (including
removing the blkcipher example) replaces the references to ablkcipher
and blkcipher with skcipher.
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>