dt-bindings: crypto: Mention clocks for mxs-dcp

Explicit clock enabling is required on 6sll and 6ull so mention that
standard clock bindings are used.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>