gpiolib: make fwnode_get_named_gpiod() static

There are no external users of fwnode_get_named_gpiod() anymore, so
let's stop exporting it and mark it as static.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>