pinctrl: rockchip：Add input schmitt support

To prevent external signal crosstalk, some pins need to
enable input schmitt, like i2c pins, 32k-input pin and so on.

Signed-off-by: David Wu <david.wu@rock-chips.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>