DT: Add vendor prefix for Integrated Silicon Solutions Inc.

ISSI is the stock ticker Integrated Silicon Solutions Inc.
Company website: http://www.issi.com

Signed-off-by: David Rivshin <drivshin@allworx.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -120,6 +120,7 @@ intercontrol	Inter Control Group
 invensense	InvenSense Inc.
 isee	ISEE 2007 S.L.
 isil	Intersil
+issi	Integrated Silicon Solutions Inc.
 jedec	JEDEC Solid State Technology Association
 karo	Ka-Ro electronics GmbH
 keymile	Keymile GmbH