mb/google/guybrush/var/nipperkin: config eSPI alert as in-band

To prevent unexpected alert from eSPI to SOC, configure this alert pin
to in-band.

BUG=b:199458949,b:203446084
BRANCH=guybrush
TEST=emerge-guybrush coreboot chromeos-bootimage

Signed-off-by: Kevin Chiu <kevin.chiu@quantatw.com>
Change-Id: I18d38fe504bd9f2069b9977d5a35729691f672d1
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57976
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Rob Barnes <robbarnes@google.com>

src/mainboard/google/guybrush/variants/nipperkin/overridetree.cb

@@ -167,4 +167,6 @@ chip soc/amd/cezanne
 			device generic 0.1 on end
 	end
 
+	register "common_config.espi_config.alert_pin" = "ESPI_ALERT_PIN_IN_BAND"
+
 end	# chip soc/amd/cezanne