FAFT: split wp_voltage and spi_voltage in FAFT config

The write protect reference voltage and SPI voltage are not necessarily
the same. Make them two separate options so that we can configure them
to be different.

BUG=chrome-os-partner:29210
TEST=Grep 'vref' and check each one is using the correct config value.

Change-Id: I5cc30ea364f482a0febd66bfd99d857630bfa401
Signed-off-by: Vic Yang <victoryang@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/202381
Reviewed-by: Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: David Hendricks <dhendrix@chromium.org>
diff --git a/server/cros/servo/firmware_programmer.py b/server/cros/servo/firmware_programmer.py
index 0b20509..8d30a71 100644
--- a/server/cros/servo/firmware_programmer.py
+++ b/server/cros/servo/firmware_programmer.py
@@ -163,7 +163,7 @@
         faft_config = FAFTConfig(self._servo.get_board())
         self._fw_path = path
         self._servo_prog_state = (
-            'spi2_vref:%s' % faft_config.wp_voltage,
+            'spi2_vref:%s' % faft_config.spi_voltage,
             'spi2_buf_en:on',
             'spi2_buf_on_flex_en:on',
             'spi_hold:off',