pinctrl: mediatek: use builtin_platform_driver

Use builtin_platform_driver() helper to simplify the code.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Acked-by: Hongzhou Yang <hongzhou.yang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>