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
1 file changed