of: Add vendor prefix for Cadence

Drivers like clocksource/cadence_ttc and net/macb already use the 'cdns'
prefix for Cadence IP.
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -16,6 +16,7 @@ avago	Avago Technologies
 bosch	Bosch Sensortec GmbH
 brcm	Broadcom Corporation
 cavium	Cavium, Inc.
+cdns	Cadence Design Systems Inc.
 chrp	Common Hardware Reference Platform
 cirrus	Cirrus Logic, Inc.
 cortina	Cortina Systems, Inc.