dt-bindings: pinctrl: sunplus,sp7021-pinctrl: reference generic schema

Reference generic pin controller schema to enforce proper root node
name.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220607121335.131497-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>