In order to aid debugging, we add a debugfs interface to the driver
that allows direct interaction with the AVS co-processor.

The debugfs interface provides a means for reading all and writing some
of the mailbox registers directly from the shell prompt and enables a
user to execute the communications protocol between ARM CPU and AVS CPU
step-by-step.

This interface should be used for debugging purposes only.
Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>