F2A85-M - amdgpu fails, integrated GPU works fine

Hello,
is there a known solution to my problem?
After flashing the coreboot and booting the computer, I can use the integrated GPU through VGA, but initialization of amdgpu fails and DisplayPort external GPU isn't recognized by the system. It appears in lspci, though.

I can provide better logs tommorow, but that's part of what appeared in dmesg:
amdgpu get bioss error acpi vcft image header truncated
amdgpu kernel bo map failed [...] error -22
amdgpu_vram_scratch_init failed [...] error -22
fatal error in GPU initialization

My specs:
F2A85-M
A10-6800k with integrated Radeon HD 8670D
AMD RX 570 a external GPU
I've used coreboot with Seabios

If there are some known solutions to the problem, please let me know. Otherwise I could use a list of files and command outputs I should provide to give better information. Thank you for your help.