lib/vsprintf: Make use of fwnode API to obtain node names and separators

Instead of implementing our own means of discovering parent nodes, node
names or counting how many parents a node has, use the newly added
functions in the fwnode API to obtain that information.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>