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>

Author: liambeguin

Documentation/devicetree/bindings/gpio/gpio-pca95xx.yaml

@@ -66,6 +66,7 @@ properties:
               - ti,tca6408
               - ti,tca6416
               - ti,tca6424
+              - ti,tca9538
               - ti,tca9539
               - ti,tca9554