crypto: doc - crypto API high level spec

The design of the kernel crypto API as well as hints to program with
the kernel crypto API are given.

The documentation contains:
 * design aspects of crypto API
 * develper specific hints
 * references to the API function description
 * source code examples

CC: Marek Vasut <marex@denx.de>
Signed-off-by: NStephan Mueller <smueller@chronox.de>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>