dt-bindings: gpio: convert bindings for NXP PCA953x family to dtschema

Convert the NXP PCA953x family of GPIO expanders bindings to device tree
schema.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200916155715.21009-2-krzk@kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>