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

Apparently RK3588 pinctrl has 13 different device functions, but dt-validate
only checks for pin configuration being referenced so I did not notice.

Fixes: ed1f77b78322 ("dt-bindings: pinctrl: rockchip: increase max amount of device functions")
Signed-off-by: Sebastian Reichel <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Acked-by: Heiko Stuebner <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Linus Walleij <[e-mail]>

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

···
               description:
                 Pin bank index.
             - minimum: 0
-              maximum: 10
+              maximum: 13
               description:
                 Mux 0 means GPIO and mux 1 to N means
                 the specific device function.