dt-bindings: arm: Add entry for Beckhoff CX9020

- add vendor prefix bhf for Beckhoff
- add new board binding bhf,cx9020

Signed-off-by: Patrick Bruenn <p.bruenn@beckhoff.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>