rockchip: rk3399: Add Rockpro64 board support

rockchip: rk3399: Add Rockpro64 board support

Add initial support for Rockpro64 board.

Specification
- Rockchip RK3399
- 2/4GB Dual-Channel LPDDR3
- SD card slot
- eMMC socket
- 128Mb SPI Flash
- Gigabit ethernet
- PCIe 4X slot
- WiFI/BT module socket
- HDMI In/Out, DP, MIPI DSI/CSI, eDP
- USB 3.0, 2.0
- USB Type C power and data
- GPIO expansion ports
- DC 12V/2A

Commit details of rk3399-rockpro64.dts sync from Linux 5.1-rc2:
"arm64: dts: rockchip: rockpro64 dts add usb regulator"
(sha1: 6db644c79c8d45d73b56bc389aebd85fc3679beb)

'Akash' has sent an initial patch before, so I keep him as board
maintainer and I'm co-maintainer based on our conversation.

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>