When I boot with the propietary BIOS I can get GRUB2 and linux console on the serial port, but only at 38400 bps. 118000 does not work. So I've put 38400 in kconfig too.
Hm this is strange. Maybe you can try with 9600 ? I checked your superiotool dump and it seems that you dont need to call it8712f_24mhz_in check if you do not call it by accident.
I would suggest to try to make it work with serialICE first, then fix the coreboot console as second step. Go to www.serialice.com and download it.
It is some kind of simple monitor which can execute various IO operations through serial port.
I'm attaching a file which could do the trick for you. Set the baudspeed to 38400 and select that asrock board in kconfig. Replace the existing file with attached, and compile. Not sure if padding is there so if you get 64KB image then you need just to place the image in last 64KB of the flash.
Then use minicom or whatever and see if you get the serialice prompt. If so, you just need to fix the coreboot to do same thing.
Thanks, Rudolf