pinctrl: nuvoton: Convert to use struct group_desc

The pin control core header provides struct group_desc.
Utilize it instead of open coded variants in the driver.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/20250318105932.2090926-5-andriy.shevchenko@linux.intel.com


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