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>

Documentation/devicetree/bindings/arm/bhf.txt

+Beckhoff Automation Platforms Device Tree Bindings
+--------------------------------------------------
+
+CX9020 Embedded PC
+Required root node properties:
+    - compatible = "bhf,cx9020", "fsl,imx53";

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -47,6 +47,7 @@ avic	Shanghai AVIC Optoelectronics Co., Ltd.
 axentia	Axentia Technologies AB
 axis	Axis Communications AB
 bananapi BIPAI KEJI LIMITED
+bhf	Beckhoff Automation GmbH & Co. KG
 boe	BOE Technology Group Co., Ltd.
 bosch	Bosch Sensortec GmbH
 boundary	Boundary Devices Inc.