rockchip: rk3399: Add Nanopi M4 board support

rockchip: rk3399: Add Nanopi M4 board support

Add initial support for Nanopi M4 board.

Specification
- Rockchip RK3399
- Dual-Channel 4GB LPDDR3-1866
- SD card slot
- eMMC socket
- RTL8211E 1Gbps
- AP6356S WiFI/BT
- HDMI In/Out, DP, MIPI DSI/CSI
- USB 3.0 x4
- USB Type C power and data
- GPIO1, GPIO2 expansion ports
- DC5V/3A

Commit details of rk3399-nanopi-m4.dts sync from Linux 5.1-rc2:
"arm64: dts: rockchip: Refine nanopi4 differences"
(sha1: c62ffaf5026d0b7633e62b2cea8450b5543c349a)

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>