dt-bindings: arm: hisilicon: add bindings for HiKey960 board

Add bindings for HiKey960 Board.

Signed-off-by: Guodong Xu <guodong.xu@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>