dt-bindings: gpio: pca9570: Add label property

Add a label property to allow a custom name to be used for identifying
a device on the board. This is useful when multiple devices are present on
the same board. Similar change was done by commit ffae65fb


("dt-bindings: spi: spi-cadence: Add label property") or by commit
a53faa6bfa3b ("dt-bindings: hwmon: ina2xx: Add label property").

Signed-off-by: Michal Simek <michal.simek@amd.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>