FAFT: Add method to control write protect pin

We need to assert/deassert hardware write protect pin to test write
protect functionality.

BUG=chrome-os-partner:12950
TEST=Verified we can enable/disable write protect.

Change-Id: Ib9b847a71c353dd5ac3353694eb37c019cd82869
Signed-off-by: Vic Yang <victoryang@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/31494
Reviewed-by: Mike Truty <truty@chromium.org>
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
diff --git a/server/cros/faftsequence.py b/server/cros/faftsequence.py
index 0293972..4ff9592 100644
--- a/server/cros/faftsequence.py
+++ b/server/cros/faftsequence.py
@@ -720,6 +720,17 @@
             })
 
 
+    def enable_write_protect(self, enable):
+        """Set hardware write protect pin.
+
+        Args:
+          enable: True if asserting write protect pin. Otherwise, False.
+        """
+        self.servo.set('fw_wp_vref', self.client_attr.wp_voltage)
+        self.servo.set('fw_wp_en', 'on')
+        self.servo.set('fw_wp', 'on' if enable else 'off')
+
+
     def send_ctrl_d_to_dut(self):
         """Send Ctrl-D key to DUT."""
         if self._customized_ctrl_d_key_command: