dt-bindings: gpio: snps,dw-apb: allow gpio-line-names

Allow the GPIO controller subnode to define GPIO names.  Already used in
at least on DTS:

  bitmain/bm1880-sophon-edge.dtb: gpio@50027000: gpio-controller@0: 'gpio-line-names' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Serge Semin <fancer.lancer@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>