arm64: dts: rockchip: add support for ROC-RK3399-PC board

ROC-RK3399-PC is a power efficient 4GB LPDDR4 single board
computer with USB 3.0 and Gigabit Ethernet in a form factor
compatible with the Raspberry Pi. It is based on the Rockchip
RK3399 SoC, powered by the Type-C port.

The devicetree currently supports peripherals of:

- Ethernet
- HDMI
- SD Card
- UART2 debug
- Type-C
- eMMC

USB3 in Type-C port currently only works with normal orientation,
not flip one.

Signed-off-by: Levin Du <djw@t-chip.com.cn>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>