rockchip: rk3399: Add common Rock960 family from Vamrs

Rock960 is a family of boards based on Rockchip RK3399 SoC from Vamrs.
It consists of Rock960 (Consumer Edition) and Ficus (Enterprise Edition)
96Boards.

Below are some of the key differences between both Rock960 and Ficus
boards:

1. Different host enable GPIO for USB
2. Different power and reset GPIO for PCI-E
3. No Ethernet port on Rock960

The common board support will be utilized by both boards. The device
tree has been organized in such a way that only the properties which
differ between both boards are placed in the board specific dts and
the reset of the nodes are placed in common dtsi file.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
[Added instructions for SD card boot]
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Peter Robinson <pbrobinson@gmail.com>