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 <[email protected]>
Acked-by: Rob Herring <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Linus Walleij <[email protected]>

Author: Marek Vasut

Documentation/devicetree/bindings/pinctrl/st,stm32-pinctrl.yaml

@@ -71,6 +71,7 @@ patternProperties:
         maxItems: 1
       resets:
         maxItems: 1
+      gpio-line-names: true
       gpio-ranges:
         minItems: 1
         maxItems: 16