pinctrl: realtek: Add common pinctrl driver for Realtek DHC RTD SoCs

The RTD SoCs share a similar design for pinmux and pinconfig.
This common pinctrl driver supports different variants within the RTD
SoCs.

Signed-off-by: Tzuyi Chang <tychang@realtek.com>
Link: https://lore.kernel.org/r/20230919101117.4097-2-tychang@realtek.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>