pca953x: add example how to use interrupt-controller and gpio-controller

It is not completely obvious that these are required and
how to use them. So we provide a tested example.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>