servo: add 1.8V supply for ZerbleBarn

Mimick the DragonTalon configuration and use the spi2_vref as the 1.8V
power supply for the sensors (in the case with a flying wire from J13
pin 5)

BUG=none
TEST=run servod with 'sudo servod -b zerblebarn -c zerblebarn_inas.xml'
a fingerprint capture with 'fpcapture' on the serial console works,
'dut-control pp1800_fpc_mv' returns something close to 1.8V.

Change-Id: I00fa6c2febe0035f546678c63c237699b6f83606
Reviewed-on: https://chromium-review.googlesource.com/1097319
Commit-Ready: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
diff --git a/servo/data/servo_zerblebarn_overlay.xml b/servo/data/servo_zerblebarn_overlay.xml
index a923cde..1b6de28 100644
--- a/servo/data/servo_zerblebarn_overlay.xml
+++ b/servo/data/servo_zerblebarn_overlay.xml
@@ -21,6 +21,11 @@
     <params clobber_ok="" init="pp3300"></params>
   </control>
   <control>
+    <name>spi2_vref</name>
+    <doc>Used as 1.8V rail for the sensor.</doc>
+    <params clobber_ok="" init="pp1800"></params>
+  </control>
+  <control>
     <name>jtag_vref_sel0</name>
     <params clobber_ok="" init="pp3300"></params>
   </control>