dt-bindings: vendor-prefixes: add ABB

Document binding for ABB.

Cc: Heiko Schocher <hs@denx.de>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>