ZTE platforms device tree bindings

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-  ZX296702 board:
    Required root node properties:
      - compatible = "zte,zx296702-ad1", "zte,zx296702"

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-  ZX296718 SoC:
    Required root node properties:
      - compatible = "zte,zx296718"

ZX296718 EVB board:
      - "zte,zx296718-evb"