gpiolib: Unify the checks on fwnode type

We have (historically) different approaches how we identify the type
of a given fwnode. Let's standardize them across the library code.

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