dt-bindings: gpio: pca953x: Document interrupts, update example

The device can optionally supply an interrupt, hence document that.

Add required GPIO properties to the example (extracted from a patch by
Wolfram Sang).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>