pinctrl: nuvoton: Convert to use struct pingroup and PINCTRL_PINGROUP()

The pin control header provides struct pingroup and PINCTRL_PINGROUP() macro.
Utilize them instead of open coded variants in the driver.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/20250318105932.2090926-3-andriy.shevchenko@linux.intel.com


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