dt-bindings: Add vendor prefix for Bticino

Added 'bticino' vendor prefix for Bticino International.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Simone CIANNI <simone.cianni@bticino.it>
Signed-off-by: Raffaele RECALCATI <raffaele.recalcati@bticino.it>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>