dt-bindings: vendor-prefixes: Add Silex Insight vendor prefix

Silex Insight is a microelectronic company whose headquarter is located
in Belgium.
Web site of the company: https://www.silexinsight.com/



Signed-off-by: Olivier Sobrie <olivier.sobrie@silexinsight.com>
Signed-off-by: Rob Herring <robh@kernel.org>