If you ever get your hands on an APU board from PCEngines and that you have an issue like this when trying to boot OpenBSD:
Entry point at 0xffffffff8100100
There is a simple solution explained by Mischa on the email@example.com mailing list in 2020.
I'll copy the reply here in case the archives get lost. When you get the OpenBSD boot prompt, type the following commands to tell about the serial port.
stty com0 115200 set tty com0 boot
And you are done! During the installation process you will be asked about serial devices to use but the default offered will match what you set at boot.