gpio: dwapb: correct gpio-cells in binding document

The example uses gpio-cells = 1 while it should be two (it is even
mentioned in the text above).

Cc: devicetree@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>