crypto: mv_cesa - use gen_pool to reserve the SRAM memory region

The mv_cesa driver currently expects the SRAM memory region to be passed
as a platform device resource.

This approach implies two drawbacks:
- the DT representation is wrong
- the only one that can access the SRAM is the crypto engine

The last point is particularly annoying in some cases: for example on
armada 370, a small region of the crypto SRAM is used to implement the
cpuidle, which means you would not be able to enable both cpuidle and the
CESA driver.

To address that problem, we explicitly define the SRAM device in the DT
and then reference the sram node from the crypto engine node.

Also note that the old way of retrieving the SRAM memory region is still
supported, or in other words, backward compatibility is preserved.

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