device property: Advertise fwnode and device property count API calls

The documentation of fwnode and device property array API calls isn't
pointing out to the shortcuts to count the number of elements of size
in an array. Amend the documentation to advertise fwnode and device
property count API calls.

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