arm64: dts: hisilicon: hikey970: Add GPIO line names

Add GPIO line names for HiSilicon HiKey970 board based on HI3670 SoC.
The Line names are derived from "hikey970-schematics.pdf" document and
named in conjunction with 96Boards CE Specification v1.0.

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