dt-bindings: pinctrl: Document nuvoton ma35d1 pin control

Add documentation to describe nuvoton ma35d1 pin control and GPIO.

Signed-off-by: Jacky Huang <ychuang3@nuvoton.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240521012447.42211-3-ychuang570808@gmail.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>