[autotest] system_facade_native: Add 'sched' to valid CPU governor mode

This is a new mode on kevin board.

BUG=None
TEST=run audio_AudioBasicHDMI test on kevin

Change-Id: If635ee19be576a39a919d7faf25d403c29df88a7
Reviewed-on: https://chromium-review.googlesource.com/487788
Commit-Ready: Cheng-Yi Chiang <cychiang@chromium.org>
Tested-by: Cheng-Yi Chiang <cychiang@chromium.org>
Reviewed-by: Wai-Hong Tam <waihong@google.com>
Reviewed-by: Kalin Stoyanov <kalin@chromium.org>
diff --git a/client/cros/multimedia/system_facade_native.py b/client/cros/multimedia/system_facade_native.py
index 9f7ae76..74f369c 100644
--- a/client/cros/multimedia/system_facade_native.py
+++ b/client/cros/multimedia/system_facade_native.py
@@ -25,6 +25,7 @@
             'performance',
             'ondemand',
             'powersave',
+            'sched'
             ]
 
     def set_scaling_governor_mode(self, index, mode):