crypto: marvell - Move SRAM I/O operations to step functions

Currently the crypto requests were sent to engines sequentially.
This commit moves the SRAM I/O operations from the prepare to the step
functions. It provides flexibility for future works and allow to prepare
a request while the engine is running.

Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>