Hi.
Is mainline support for U-Boot planned for 3C ?
The same goes for 6.x kernel ?
Is DTS from 5.x line suitable for use with 6.x ?
ps. why there’s no subcategory for 3C ? 
Mainline U-Boot / kernel support for ROCK 3C
Is DTS from 5.x line suitable for use with 6.x ?
You have to write a devicetree for 6.x mainline kernel.
As @amazingfate mention, someone first have to upstream a device tree for ROCK 3C to mainline linux.
After it has been accepted and merged into linux-rockchip maintainer tree, that device tree can then be used/imported to create a proper target in mainline U-Boot.
Fortunately the Quartz64-B schematics is very similar to ROCK 3C, I use this device tree for my mainline ROCK 3C needs 
See https://github.com/Kwiboo/u-boot-rockchip/commit/06caaa0f8cc6f10d6818da0d02f43c0841b8ca83
Why Radxa can’t write proper dts file for their product?