MAINTAINERS: Add entry for the software nodes

Making Andy and myself (Heikki) the designated reviewers of
the thing. The software node mailing list shall be
linux-acpi@vger.kernel.org for now.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>