greybus: svc_watchdog: Add sysfs file to change the behavior of bite

Currently, AP performs unipro_reset if SVC fails to response to its
ping. While this error recovery is best suited for the end-user
experience, errors in the UniPro network could potentially go unnoticed
by the QA and fishfooders in the development phase of the project. This
patch adds an option to trigger a kernel panic so logs can be collected
for analysis.

Testing Done:
 - Reproduce issue and observe kernel panic when
   watchdob_control is changed to 'panic'

Signed-off-by: David Lin <dtwlin@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>