gpio: pca9570: Add DT bindings for NXP PCA9571

This patch adds device tree bindings for the NXP PCA9571,
a 8-bit I2C GPIO expander.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>