device property: rename helper functions

To be in align with the rest of fwnode types we rename the built-in property
set ones, i.e.
	is_pset() -> is_pset_node()
	to_pset() -> to_pset_node()

There is no functional change.

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