dt-bindings: pinctrl: st,stm32: Document gpio-line-names

dt-bindings: pinctrl: st,stm32: Document gpio-line-names

Document gpio-line-names property as valid property.
This fixes dtbs_check warnings when building current Linux DTs:

"
arch/arm/boot/dts/stm32mp153c-dhcom-drc02.dtb: pinctrl@50002000: gpio@50009000: 'gpio-line-names' does not match any of the regexes: 'pinctrl-[0-9]+'
"

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220926204724.381760-1-marex@denx.de
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>