arm64: dts: rockchip: add support for firefly-rk3399 board

Firefly-rk3399 is a bord from T-Firefly, you can find detail about
it here:
http://en.t-firefly.com/en/firenow/Firefly_RK3399/



This patch add basic node for the board and make it able to bring
up.

Peripheral works:
- usb hub which connect to ehci controller;
- UART2 debug
- eMMC
- PCIe

Not work:
- USB 3.0 HOST, type-C port
- sdio, sd-card

Not test for other peripheral:
- HDMI
- Ethernet
- OPTICAL
- WiFi/BT
- MIPI CSI/DSI
- IR
- EDP/DP

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>