rockchip: add support for Radxa ROCK 3A

rockchip: add support for Radxa ROCK 3A

Radxa ROCK 3A is a credit card sized SBC with rich interfaces[1] using
the Rockchip RK3328 SoC.

Hardware
--------
- Rockchip RK3568 SoC
- Quad Cortex-A55 CPU
- Mail G52 GPU
- 1 TOPs NPU
- 2/4/8GB LPDDR4 RAM
- 1x eMMC connector
- 1x M.2 M Key (2-lane PCIe 3.0) supporting NVMe SSD
- 1x Micro SD card slot
- 1x 1000M Ethernet with PoE support (additional PoE HAT required)
- 1x USB 3.0 Type-A port (Host)
- 1x USB 3.0 Type-A port (OTG)
- 2x USB 2.0 Type-A ports (Host)
- 40 pin color GPIO header

[1] https://radxa.com/products/rock3/3a

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/15985
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>