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: NLeonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: NFabio Estevam <festevam@gmail.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/crypto/fsl-dcp.txt

@@ -6,6 +6,8 @@ Required properties:
 - interrupts : Should contain MXS DCP interrupt numbers, VMI IRQ and DCP IRQ
                must be supplied, optionally Secure IRQ can be present, but
 	       is currently not implemented and not used.
+- clocks : Clock reference (only required on some SOCs: 6ull and 6sll).
+- clock-names : Must be "dcp".
 
 Example: