Hmmm, no.
I tried to add this manually, creating a patch with this command:
./compile.sh CREATE_PATCHES=yes BOARD=rock-3c BRANCH=current uboot-patch
But when I looked the file rk3566-rock-3c.dts is already present.
AFAIK It gets brought in here:
[🌱] Fetching SHA1 of 'branch' 'refs/heads/stable-4.19-rock3' [ https://github.com/radxa/u-boot.git ]
So I guess there needs to be something else get the dtb file compiled. Not found it yet.