pinctrl/lantiq: only probe available pad controllers

The template falcon.dtsi lists all 6 pad controllers that
can be loaded. Only probe those that have status = "okay";
inside the dts file.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>