gpiolib: swnode: Introduce swnode_gpio_get_reference() helper

Instead of the spreading simlar code over the file, introduce a helper.
It also enforces the nargs validation for all GPIO software node APIs.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240819142945.327808-4-andriy.shevchenko@linux.intel.com


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>