dt-bindings: gpio: aspeed,sgpio: Specify gpio-line-names

dt-bindings: gpio: aspeed,sgpio: Specify gpio-line-names

Some devicetrees specify gpio-line-names in the sgpio node despite it
not being defined by the binding. It's a reasonable thing to do, so
define the property to squash warnings such as:

    arch/arm/boot/dts/aspeed/aspeed-bmc-vegman-rx20.dtb: sgpio@1e780200: 'gpio-line-names' does not match any of the regexes: 'pinctrl-[0-9]+'

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Link: https://lore.kernel.org/r/20240530-dt-warnings-gpio-sgpio-interrupt-cells-v2-1-912cd16e641f@codeconstruct.com.au
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>