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 <[e-mail]>
Signed-off-by: Nicolas Frattaroli <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Reviewed-by: Heiko Stuebner <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Linus Walleij <[e-mail]>

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

···
               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.