as the title says I've already opened a draft PR to add support for Radxa CM3 IO board in https://github.com/openwrt/openwrt/pull/14436
The board is already supported in upstream for over an year now so it just needs some openwrt love 
As you can also read in the PR's comments the build is failing on image building stage due to some uboot building issue.
# Copy the idbloader and the u-boot image to the image at sector 0x40 and 0x4000
dd if="/data/openwrt-gh-maurerr/staging_dir/target-aarch64_generic_musl/image"/radxa-cm3-io-3566-idbloader.img of="/data/openwrt-gh-maurerr/build_dir/target-aarch64_generic_musl/linux-rockchip_armv8/tmp/openwrt-rockchip-armv8-radxa_cm3-io-ext4-sysupgrade.img.gz" seek=64 conv=notrunc
dd: failed to open '/data/openwrt-gh-maurerr/staging_dir/target-aarch64_generic_musl/image/radxa-cm3-io-3566-idbloader.img': No such file or directory
make[5]: *** [Makefile:62: /data/openwrt-gh-maurerr/build_dir/target-aarch64_generic_musl/linux-rockchip_armv8/tmp/openwrt-rockchip-armv8-radxa_cm3-io-ext4-sysupgrade.img.gz] Error 1
Does someone has any idea how to fix rk3566 boot ?
thanks