device property: Add helpers to count items in an array

The usual pattern to allocate the necessary space for an array of properties is
to count them first by calling:

  count = device_property_read_uXX_array(dev, propname, NULL, 0);
  if (count < 0)
	return count;

Introduce helpers device_property_count_uXX() to count items by supplying hard
coded last two parameters to device_property_readXX_array().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>