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.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>