mb/google/rex: Add chip config for gspi devices

+-----------+-------------+------------------+
| INTERFACE | PCI (B:D:F) | DEVICE           |
+-----------+-------------+------------------+
| GSPI-0    | 0:0x1e:2    | NA               |
+-----------+-------------+------------------+
| GSPI-1    | 0:0x1e:3    | Finger Print MCU |
+-----------+-------------+------------------+
| GSPI-2    | 0:0x12:6    | NA               |
+-----------+-------------+------------------+

BUG=b:224325352
TEST=Able to build Google/Rex and boot to emulator.

Signed-off-by: Subrata Banik <subratabanik@google.com>
Change-Id: I4b20e342cbca60821f82c07f72328cf63c0e5404
Reviewed-on: https://review.coreboot.org/c/coreboot/+/65763
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>

src/mainboard/google/rex/variants/rex0/overridetree.cb

@@ -1,5 +1,11 @@
 chip soc/intel/meteorlake
 
+	register "serial_io_gspi_mode" = "{
+		[PchSerialIoIndexGSPI0] = PchSerialIoDisabled,
+		[PchSerialIoIndexGSPI1] = PchSerialIoPci,
+		[PchSerialIoIndexGSPI2] = PchSerialIoDisabled,
+	}"
+
 	register "serial_io_i2c_mode" = "{
 		[PchSerialIoIndexI2C0] = PchSerialIoPci,
 		[PchSerialIoIndexI2C1] = PchSerialIoPci,