poppy: add servo jtag vref control

Certain poppy variants need the uart3 reference voltage to be supplied
by servo. This change sets jtagvref to 3.3V for poppy.

BUG=b:62606493
BRANCH=none
TEST=none

Change-Id: I1e35361d214e850909e7be3df1e232c2ff59a853
Signed-off-by: Mary Ruthven <mruthven@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/563703
diff --git a/servo/data/servo_poppy_overlay.xml b/servo/data/servo_poppy_overlay.xml
index b8623b9..4d587c4 100644
--- a/servo/data/servo_poppy_overlay.xml
+++ b/servo/data/servo_poppy_overlay.xml
@@ -39,4 +39,13 @@
     <params subtype="volume_down_gpio" interface="10" drv="ec" map="yesno"
     clobber_ok=""></params>
   </control>
+  <!-- cr50 UART Buffers & VREF Controls -->
+  <control>
+    <name>jtag_vref_sel0</name>
+    <params clobber_ok="" init="pp3300"></params>
+  </control>
+  <control>
+    <name>jtag_vref_sel1</name>
+    <params clobber_ok="" init="pp3300"></params>
+  </control>
 </root>