arm64: dts: Add devicetree support for HiKey970 board

Add devicetree support for HiKey970 development board which
based on Hi3670 SoC and is also one of the 96Boards Consumer
Edition and AI platform.

Only UART6 is enabled which is the default console required
by the 96Boards Consumer Edition Specification.

This patch has been tested on HiKey970 Board.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>