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: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>