veyron: add a custom servo power driver for Rialto

Rialto can not be powered down through Servo, so this will simulate power_off
by holding the device in cold reset

With the patch Rialto can pass the platform_ServoPowerStateController_USBPluggedin test.

BUG=none
TEST=cros_run_bvt --labqual

Change-Id: I0d9eac4c3d6ce7bb2f3a268cde439ed3fff9f735
Reviewed-on: https://chromium-review.googlesource.com/295565
Commit-Ready: Jason Simmons <jsimmons@chromium.org>
Tested-by: Jason Simmons <jsimmons@chromium.org>
Reviewed-by: Jason Simmons <jsimmons@chromium.org>
diff --git a/servo/servo_server.py b/servo/servo_server.py
index b367b81..4f5e12c 100755
--- a/servo/servo_server.py
+++ b/servo/servo_server.py
@@ -134,7 +134,8 @@
       return keyboard_handlers.StoutHandler(servo)
     elif board in ('buddy', 'cranky', 'guado', 'jecht', 'mccloud', 'monroe',
                    'ninja', 'nyan_kitty', 'panther', 'rikku', 'stumpy',
-                   'sumo', 'tidus', 'tricky', 'veyron_mickey', 'zako'):
+                   'sumo', 'tidus', 'tricky', 'veyron_mickey', 'veyron_rialto',
+                   'zako'):
       if self._usbkm232 is None:
         logging.warn("No device path specified for usbkm232 handler. Returning "
                      "the MatrixKeyboardHandler, which is likely the wrong "