dt-bindings: Add vendor prefix for Hirschmann

Hirschmann is building devices for automation and networking. Add them to the
vendor prefixes.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>