device: property: add helpers to count items in string arrays

Instead of doing the following:

    count = device_property_read_string_array(dev, propname, NULL, 0);

Let's provide inline helpers with hardcoded arguments for counting
strings in property arrays.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>