gpiolib: add support for scope-based management to gpio_device

As the few users that need to get the reference to the GPIO device often
release it right after inspecting its properties, let's add support for
the automatic reference release to struct gpio_device.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>