dts: add support for Rockchip rk3399 soc

These files are from kernel upstream:
"649a371 Add linux-next specific files for 20160616"
with some modification need by U-Boot:
- chosen with stdout-path to uart2.
- add clock-frequency for uart2

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>