software nodes: Split software_node_notify()

Split software_node_notify_remove) out of software_node_notify()
and make device_platform_notify() call the latter on device addition
and the former on device removal.

While at it, put the headers of the above functions into base.h,
because they don't need to be present in a global header file.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>