gpiolib: of: Allow "trigger-sources" to reference a GPIO

The "trigger-sources" phandle used for LED triggers are special:
the DT bindings mandate that such triggers have the same phandle
references no matter what the trigger is. A GPIO is just another
kind of device that can trigger a LED.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>