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>

Author: hnez

Documentation/devicetree/bindings/gpio/gpio-pca9570.yaml

@@ -24,6 +24,10 @@ properties:
   '#gpio-cells':
     const: 2
 
+  gpio-line-names:
+    minItems: 4
+    maxItems: 8
+
 required:
   - compatible
   - reg