rockchip: add support for Radxa ROCK Pi S

rockchip: add support for Radxa ROCK Pi S

Radxa ROCK Pi S is a small in size, full in features SBC[1] using the
Rockchip RK3308B SoC.

Hardware
--------
- Rockchip RK3308B SoC
- Quad A35 CPU
- 256/512MB DDR3 RAM
- Optional 4/8GB eMMC
- Micro SD Card slot
- Optional WiFi 4 and BT 4 (not supported yet)
- 1x 100M Ethernet with PoE support (additional PoE HAT required)
- 1x USB 2.0 Type-A port (Host)
- 1x USB 2.0 Type-C port (OTG)
- 2x 26 Pin GPIO header

[1] https://radxa.com/products/rockpi/pis

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

Signed-off-by: FUKAUMI Naoki <naoki@radxa.com>
Link: https://github.com/openwrt/openwrt/pull/15933
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>