crypto: mv_cesa - document the clocks property

On Dove platforms, the crypto engine requires a clock. Document this
clocks property in the mv_cesa bindings doc.
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/crypto/mv_cesa.txt

@@ -7,6 +7,8 @@ Required properties:
         length
 - reg-names : "regs" , "sram";
 - interrupts : interrupt number
+- clocks: reference to the crypto engines clocks. This property is only
+	  required for Dove platforms
 
 Examples: