here one solution was to set the fw_setenv tdxargs "clk-imx8mm.mcore_booted=1" as documented here.
Maybe this could be the solution in my case. However this variable has to be set to a device-depenent value - clk-imx8mm.mcore_booted is for the iMX8MM.
Do you know what the equivalent value would be for the iMX8QXP or iMX8X?