dt-bindings: gpio: pca95xx: document new tca9538 chip

Add a compatible for tca9538.

Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
[Bartosz: tweaked the commit message]
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>