pinctrl: pinconf-generic: add "input-schmitt" DT property

PIN_CONFIG_INPUT_SCHMITT is defined in enum_pin_config_param,
but the corresponding DT property is missing.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>