mb/google/rex/variants/ovis: Use correct device_index for RT8168

Fix ethernet MAC address configuration. Currently, coreboot would
use ethernet_mac0 for both ports when setting the system's MAC
address. Instead, set the right device_index for the second controller
to pick up ethernet_mac1.

BUG=b:294856127
TEST=boot device and observe two different MAC addresses on the ethernet
     ports.
Change-Id: I5ff6d62d2f837a120f7095f9b9aed487e6c5aee4
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77044
Reviewed-by: Subrata Banik <subratabanik@google.com>
Reviewed-by: Jakub Czapiga <jacz@semihalf.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

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

@@ -81,7 +81,7 @@ chip soc/intel/meteorlake
 			chip drivers/net
 				register "customized_leds" = "0x05af"
 				register "wake" = "GPE0_DW0_18"
-				register "device_index" = "0"
+				register "device_index" = "1"
 				register "add_acpi_dma_property" = "true"
 				device pci 00.0 on end
 			end