dt-bindings: gpio: pca953x: Document onnn,cat9554

The ON Semiconductor CAT9554 is a variant of the PCA953x GPIO expander,
with 8 GPIOs and interrupt functionality.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NBartosz Golaszewski <bgolaszewski@baylibre.com>

Documentation/devicetree/bindings/gpio/gpio-pca953x.txt

@@ -30,6 +30,7 @@ Required properties:
 	ti,tca6424
 	ti,tca9539
 	ti,tca9554
+	onnn,cat9554
 	onnn,pca9654
 	exar,xra1202
  - gpio-controller: if used as gpio expander.