dt-bindings: gpio: fsl-imx-gpio: add gpio-line-names

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

  arch/arm/boot/dts/imx53-m53menlo.dt.yaml: gpio@53f84000:
    'gpio-line-names' does not match any of the regexes: '^(hog-[0-9]+|.+-hog(-[0-9]+)?)$', 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200920195848.27075-3-krzk@kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>