mb/google/fatcat: Keep GSPIx interface default PCI

BUG=b:377595986
TEST=Able to see 0x12.6 device is visible using `lspci`.

Change-Id: Ia3348f78614e61259333ccf2babf20eaf4666a0e
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/85686
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: YH Lin <yueherngl@google.com>

src/mainboard/google/fatcat/variants/baseboard/fatcat/devicetree.cb

@@ -51,6 +51,12 @@ chip soc/intel/pantherlake
 		[PchSerialIoIndexUART2] = PchSerialIoDisabled,
 	}"
 
+	register "serial_io_gspi_mode" = "{
+		[PchSerialIoIndexGSPI0] = PchSerialIoPci,
+		[PchSerialIoIndexGSPI1] = PchSerialIoPci,
+		[PchSerialIoIndexGSPI0A] = PchSerialIoPci,
+	}"
+
 	register "pch_hda_dsp_enable" = "true"
 	register "pch_hda_idisp_link_tmode" = "HDA_TMODE_8T"
 	register "pch_hda_idisp_link_frequency" = "HDA_LINKFREQ_96MHZ"