PM: tools: use canonical ftrace path

The canonical location for the tracefs filesystem is at /sys/kernel/tracing.

But, from Documentation/trace/ftrace.rst:

  Before 4.1, all ftrace tracing control files were within the debugfs
  file system, which is typically located at /sys/kernel/debug/tracing.
  For backward compatibility, when mounting the debugfs file system,
  the tracefs file system will be automatically mounted at:

  /sys/kernel/debug/tracing

A few scripts in tools/power still refer to this older debugfs path, so
let's update them to avoid confusion.

Signed-off-by: Ross Zwisler <zwisler@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>