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>
diff --git a/arch/arm/dts/rk3399-nanopi4-u-boot.dtsi b/arch/arm/dts/rk3399-nanopi4-u-boot.dtsi
index bb8bdf0..a126bba 100644
--- a/arch/arm/dts/rk3399-nanopi4-u-boot.dtsi
+++ b/arch/arm/dts/rk3399-nanopi4-u-boot.dtsi
@@ -3,6 +3,8 @@
  * Copyright (C) 2019 Jagan Teki <jagan@amarulasolutions.com>
  */
 
+#include "rk3399-u-boot.dtsi"
+
 &sdmmc {
 	pinctrl-0 = <&sdmmc_bus4 &sdmmc_clk &sdmmc_cmd &sdmmc_cd>;
 };