mb/google/dedede/var/taranza: Disable EXT_VR

The taranza removed the APW8738BQBI and "disable_external_bypass_vr" should be set to "1" to disable.

BUG=b:288978340
TEST=emerge-dedede coreboot

Signed-off-by: Sheng-Liang Pan <sheng-liang.pan@quanta.corp-partner.google.com>
Change-Id: I0a849fbfacba1d200c969c66bb058863d7ab3085
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76125
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
diff --git a/src/mainboard/google/dedede/variants/taranza/overridetree.cb b/src/mainboard/google/dedede/variants/taranza/overridetree.cb
index e3df1f2..fa8d409 100644
--- a/src/mainboard/google/dedede/variants/taranza/overridetree.cb
+++ b/src/mainboard/google/dedede/variants/taranza/overridetree.cb
@@ -53,6 +53,9 @@
 	register "DdiPortAHpd" = "1"
 	register "DdiPortADdc" = "1"
 
+	# Does not support external vnn power rail
+	register "disable_external_bypass_vr" = "1"
+
 	# USB Port Configuration
 	register "usb2_ports[0]" = "{
 		.enable = 1,