crypto: mxs-dcp - Fix SHA null hashes and output length

DCP writes at least 32 bytes in the output buffer instead of hash length
as documented. Add intermediate buffer to prevent write out of bounds.

When requested to produce null hashes DCP fails to produce valid output.
Add software workaround to bypass hardware and return valid output.

Signed-off-by: Radu Solea <radu.solea@nxp.com>
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>