gpio: pca9570: add pca9571 support

The PCA9571 very similar to the PCA9570, it only differs in the
number of GPIOs.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>