pinctrl: realtek: Constify struct regmap_config

`rtd_pinctrl_regmap_config` is not modified and can be declared as const
to move its data to a read-only section.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://lore.kernel.org/20240704-pinctrl-const-regmap_config-v1-2-9d5570f0b9f3@gmail.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>