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>