gpiolib: of: Move enum of_gpio_flags to its only user

GPIO library for OF is the only user for enum of_gpio_flags.
Move it there.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>