of: Add vendor prefix for Asahi Kasei Corp.

Their stock ticker is 3407.T which wouldn't make a good DT vendor
prefix. Use the company name initials instead.
Signed-off-by: NStephen Warren <swarren@nvidia.com>
Signed-off-by: NRob Herring <rob.herring@calxeda.com>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -5,6 +5,7 @@ using them to avoid name-space collisions.
 
 ad	Avionic Design GmbH
 adi	Analog Devices, Inc.
+ak	Asahi Kasei Corp.
 amcc	Applied Micro Circuits Corporation (APM, formally AMCC)
 apm	Applied Micro Circuits Corporation (APM)
 arm	ARM Ltd.