dt-bindings: Add vendor prefix for Avnet, Inc.

"Avnet, Inc. is one of the world's largest distributors of electronic
components and embedded solutions." - Wikipedia.org

Website: www.avnet.com

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Rob Herring <robh@kernel.org>