rockchip: add Radxa CM3 IO board support

Hardware
--------
RockChip RK3566 ARM64 (4 cores)
- up to 8GB LPDDR4X
- 1x HDMI,
- 2x MIPI DSI
- 2x MIPI CSI2
- 1x eDP
- 1x PCIe card
- 2x SATA
- 2x USB 2.0 Host
- 1x USB 3.0
- 1x USB 2.0 OTG
- 10/100/1000 Base-T
- microSD slot
- 40-pin GPIO expansion header
- 12V DC

Radxa CM3 needs to mount on top of this IO board in order to create
complete Radxa CM3 IO board platform.

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

Reviewed-by: Tianling Shen <cnsztl@immortalwrt.org>
Signed-off-by: Marius Durbaca <mariusd84@gmail.com>