rockchip: rk3399: Add Rock PI 4 support

rockchip: rk3399: Add Rock PI 4 support

Add initial support for Rock PI 4 board.

Specification
- Rockchip RK3399
- LPDDR4
- eMMC
- SD card slot
- RTL8211E 1Gbps
- HDMI In/Out, DP, MIPI DSI/CSI
- PCIe M.2
- USB 2.0, USB-3.0
- USB C Type

Commit details of rk3399-rock-pi-4.dts sync from Linux 5.1-rc2:
"arm64: dts: rockchip: add ROCK Pi 4 DTS support"
(sha1: 1b5715c602fda7b812af0e190eddcce2812e5417)

Signed-off-by: Akash Gajjar <akash@openedev.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>