rockchip: rk3399: Add Rock960 CE board support

rockchip: rk3399: Add Rock960 CE board support

Add board support for Rock960 CE board from Vamrs. This board utilizes
common Rock960 family support.

Following peripherals are tested and known to work:
* USB 2.0
* MMC

This commit also adds DDR configuration for LPDDR3-2GiB-1600MHz which
is being used on the board.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>