dt-bindings: gpio: pca9570: add gpio-line-names property

This patch allows giving each of the controller's pins a meaningful
name.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>