dt-bindings: gpio: pl061: add gpio-line-names

Describe common "gpio-line-names" property to fix dtbs_check warnings
like:

  arch/arm64/boot/dts/hisilicon/hi3670-hikey970.dt.yaml: gpio@e8a0b000:
    'gpio-line-names' does not match any of the regexes: 'pinctrl-[0-9]+'
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200920195848.27075-1-krzk@kernel.orgSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/pl061-gpio.yaml

@@ -51,7 +51,10 @@ properties:
 
   gpio-controller: true
 
+  gpio-line-names: true
+
   gpio-ranges:
+    minItems: 1
     maxItems: 8
 
 required: