mb/google/brya/var/vell: Disable I2C6 controller

This patch disables unused I2C6 controller for the 'vell' variant of
the 'brya' mainboard.

BUG=b:352330495
TEST=Able to build and boot google/vell.

Change-Id: I5b39e44bb64bf2285c962249c0d94a8d5325f0c7
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/84490
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <ericllai@google.com>

src/mainboard/google/brya/variants/vell/overridetree.cb

@@ -90,6 +90,7 @@ chip soc/intel/alderlake
 		[PchSerialIoIndexI2C3] = PchSerialIoPci,
 		[PchSerialIoIndexI2C4] = PchSerialIoDisabled,
 		[PchSerialIoIndexI2C5] = PchSerialIoPci,
+		[PchSerialIoIndexI2C6] = PchSerialIoDisabled,
 		[PchSerialIoIndexI2C7] = PchSerialIoPci,
 	}"
 	device domain 0 on