gpiolib: of: Use named item for enum gpiod_flags variable

Use named item instead of plain integer for enum gpiod_flags
to make it clear that even 0 has its own meaning.

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