DTS: Bindings: pca953x add an optional vcc-supply property

Hardware can have a switchable Vcc supply, so let's add it to
the bindings (the current Linux driver code already supports it).
Signed-off-by: NH. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

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

@@ -35,6 +35,7 @@ Required properties:
 Optional properties:
  - reset-gpios: GPIO specification for the RESET input. This is an
 		active low signal to the PCA953x.
+ - vcc-supply:	power supply regulator.
 
 Example: