software node: mark internal macros with double underscores

Let's mark PROPERTY_ENTRY_* macros that are internal with double leading
underscores so users are not tempted to use them.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>