board: hisilicon: Add support for Hikey960 board

This commit adds board support for Hikey960 board from Hisilicon. This
board is one of the Consumer Edition boards of the 96Boards family
powered by Kirin960 SoC.

More information about this board can be found in 96Boards website:
https://www.96boards.org/product/hikey960/

The initial supported/tested devices include:
- Debug serial
- SD

With these support, it's good enough for loading Linux Kernel from SD.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>