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