dt-bindings: gpio: pca953x: Document new supported chip pca9506

The previous patch added support for this chip. Add its name to the list
of allowed compatibles.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>