device property: Read strings using string array reading functions

Always read strings using of_property_read_string_array() instead of
of_property_read_string(). This allows using a single operation struct
callback for accessing strings.

Same for pset_prop_read_string_array() and pset_prop_read_string().

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>