dt-bindings: Add vendor prefix for Linutronix

Add a vendor prefix for Linutronix GmbH.
Website: https://linutronix.de/



Co-developed-by: Benedikt Spranger <b.spranger@linutronix.de>
Signed-off-by: Benedikt Spranger <b.spranger@linutronix.de>
Signed-off-by: Bastian Germann <bage@linutronix.de>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>