rockchip: rk3399: Add Ficus EE board support

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

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

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
[Reworked based on common Rock960 family support]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>