dt-bindings: gpio: brcmstb: drop unneeded quotes

Drop unneeded quotes over simple string values to fix a soon to be
enabled yamllint warning:

  [error] string value is redundantly quoted with any quotes (quoted-strings)
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Documentation/devicetree/bindings/gpio/brcm,brcmstb-gpio.yaml

@@ -72,7 +72,7 @@ required:
   - reg
   - gpio-controller
   - "#gpio-cells"
-  - "brcm,gpio-bank-widths"
+  - brcm,gpio-bank-widths
 
 additionalProperties: false