I found a solution:
compile the dts file
dtc -O dtb -o rk3588-add-hdptxphy_hdmi_clk.dtbo rk3588-add-hdptxphy_hdmi_clk.dts
and install it into your current kernel dtbs directory:
cp rk3588-add-hdptxphy_hdmi_clk.dtbo /boot/dtbs/uname -r/rockchip/overlay/
sudo vim /boot/extlinux/extlinux.conf
and replace /dtbs/5.10.66-27-rockchip-gea60d388902d/rockchip/overlay/rock-5b-hdmi1-8k.dtbo with /dtbs/5.10.66-27-rockchip-gea60d388902d/rockchip/overlay/rk3588-add-hdptxphy_hdmi_clk.dtbo
use uname -r to get your current kernel release string and replace it accordingly.
reboot and then change the resolution in you running X11 session.