gpio: pca9570: Add DT bindings for NXP PCA9570

This patch adds device tree bindings for the NXP PCA9570,
a 4-bit I2C GPO expander.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200630160934.1197066-1-mans0n@gorani.run


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>