dt-bindings: Add vendor prefix for Linksys

Linksys (http://www.linksys.com/us/) is a vendor of networking products
mostly for use by home and small business user. The brand is currently
owned by Belkin International, Inc. (http://www.belkin.com/us/

).

The vendor prefix linksys is already used by various supported boards.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>