arm64: rk3399: Add support NanoPi R4s

NanoPi R4s is SBC base on Rockchip RK3399 hexa-core processor with
dual-Core Cortex-A72 and Mali-T864 GPU with 4GiB(LPDDR4) of RAM, SD card support,
including 2 gigabit ethernet(RTL8211E 1Gbps - RTL8111H 1Gbps) and 2 USB 3.0 port.
port.It also has two GPIO headers which allows further peripherals to be used.

The devicetree file is taken of the rk3399 nanopi4 Linux kernel [1].

[1] https://github.com/torvalds/linux/commit/e7a095908227fb3ccc86d001d9e13c9ae2bef8e6

Signed-off-by: xiaobo <peterwillcn@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/board/rockchip/evb_rk3399/MAINTAINERS b/board/rockchip/evb_rk3399/MAINTAINERS
index 4c889e0..3b9d60e 100644
--- a/board/rockchip/evb_rk3399/MAINTAINERS
+++ b/board/rockchip/evb_rk3399/MAINTAINERS
@@ -55,6 +55,12 @@
 F:	configs/nanopi-neo4-rk3399_defconfig
 F:	arch/arm/dts/rk3399-nanopi-neo4-u-boot.dtsi
 
+NANOPI-R4S
+M:	Xiaobo Tian <peterwillcn@gmail.com>
+S:	Maintained
+F:	configs/nanopi-r4s-rk3399_defconfig
+F:	arch/arm/dts/rk3399-nanopi-r4s-u-boot.dtsi
+
 ORANGEPI-RK3399
 M:	Jagan Teki <jagan@amarulasolutions.com>
 S:	Maintained