To improve the support a bit navigate to coreboot/util/getpir/ and run make, then run ./getpir (on the GX1), this will generate a irq_tables.c file. Copy it to your folder, and add the following to the first section of your Kconfig.
select HAVE_PIRQ_TABLE
Once you have done this you should be ready to compile CoreBoot and flash it on to your board. If the flashing fails or the compiled ROM doesn't work for your board could be unable to boot, so it is strongly recommended that you find or buy a similar ROM chip so that you have a working backup in case something goes wrong.
-Anders