dt-bindings: gpio: convert bindings for Maxim MAX732x family to dtschema

Convert the Maxim MAX732x family of GPIO expanders bindings to device
tree schema by merging it with existing PCA95xx schema.  These are quite
similar so merging reduces duplication.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200916155715.21009-3-krzk@kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>