dt-bindings: gpio: sifive: add gpio-line-names

Fix device tree schema validation messages like 'gpio-line-names'
does not match any of the regexes: 'pinctrl-[0-9]+' From schema: ...
sifive,gpio.yaml'.

The bindings were missing the gpio-line-names element, which was
causing the dt-schema checker to trip-up.
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NAtul Khare <atulkhare@rivosinc.com>
Signed-off-by: NConor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220803155539.800766-1-mail@conchuod.ieSigned-off-by: NPalmer Dabbelt <palmer@rivosinc.com>

Documentation/devicetree/bindings/gpio/sifive,gpio.yaml

@@ -46,6 +46,10 @@ properties:
     maximum: 32
     default: 16
 
+  gpio-line-names:
+    minItems: 1
+    maxItems: 32
+
   gpio-controller: true
 
 required: