rockchip: add NanoPi R4S support

rockchip: add NanoPi R4S support

Hardware
--------
RockChip RK3399 ARM64 (6 cores)
4GB LPDDR4 RAM
2x 1000 Base-T
3 LEDs (LAN / WAN / SYS)
1 Button (Reset)
Micro-SD slot
2x USB 3.0 Port

Installation
------------
Uncompress the OpenWrt sysupgrade and write it to a micro SD card using
dd.

=====================================
NOTICE FOR USERS WHO USE 1GB VERSION:
     BY NOW IT IS NOT SUPPORTED
====================================

[initialed target]
Co-developed-by: Marty Jones <mj8263788@gmail.com>
Signed-off-by: Marty Jones <mj8263788@gmail.com>
[fixed bootscript]
Co-developed-by: Jayantajit Gogoi <jayanta.gogoi525@gmail.com>
Signed-off-by: Jayantajit Gogoi <jayanta.gogoi525@gmail.com>
Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>