gpio: rtd: Add support for Realtek DHC(Digital Home Center) RTD SoCs

This driver enables configuration of GPIO direction, GPIO values, GPIO
debounce settings and handles GPIO interrupts.

Signed-off-by: Tzuyi Chang <tychang@realtek.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>