dt-bindings: Add documentation for Broadcom Hurricane 2 SoCs

Broadcom Hurricane 2 device tree bindings

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Broadcom Hurricane 2 family of SoCs are used for switching control. These SoCs

are based on Broadcom's iProc SoC architecture and feature a single core Cortex

A9 ARM CPUs, DDR2/DDR3 memory, PCIe GEN-2, USB 2.0 and USB 3.0, serial and NAND

flash and a PCIe attached integrated switching engine.

Boards with Hurricane SoCs shall have the following properties:

Required root node property:

BCM53342

compatible = "brcm,bcm53342", "brcm,hr2";