For 5.10 kernel you would only need to compile the dtb file and copy it across. There is no need to change/enabel pice3x2. If your compiling on rock5b (although slow) the instructions on the wiki do work. However they do seem to rebuild the whole kernel/dtbs and repackage them everytime. I’ve used them when re-configuring pcie for testing a gpu card. To stop it rebuilding from scratch you can change a line in pack-kernel.sh to
cd ${KERNEL_DIR} && make && make -f $ROCKCHIP_BSP_DIR/build/kernel-package.mk kernel-package
If you wanting to cross compile on x86 then you need to export some environment variable to tell it to use arm gcc.