software node: Update MAINTAINERS data base

There are two updates to the MAINTAINERS regarding to software node API:
- add Daniel Scally to be designated reviewer
- add Sakari Ailus to be designated reviewer
- add rather tightly related device property files to the list
- due to above adjust section name accordingly

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: Daniel Scally <djrscally@gmail.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>