dt-bindings: pinctrl: rockchip: increase max amount of device functions

With the introduction of the RK3576, the maximum device function ID used
increased to 14, as anyone can easily verify for themselves with:

  rg -g '*-pinctrl.dtsi' '<\d+\s+RK_P..\s+(?<func>\d+)\s.*>;$' --trim \
  -NI -r '$func' arch/arm64/boot/dts/rockchip/ | sort -g | uniq

Unfortunately, this wasn't caught by dt-validate as those pins are
omit-if-no-ref and we had no reference to them in any tree so far.

Once again kick the can down the road by increasing the limit to 14.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/20250602-rk3576-pwm-v2-1-a6434b0ce60c@collabora.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/rockchip,pinctrl.yaml

@@ -135,7 +135,7 @@ additionalProperties:
               description:
                 Pin bank index.
             - minimum: 0
-              maximum: 13
+              maximum: 14
               description:
                 Mux 0 means GPIO and mux 1 to N means
                 the specific device function.