pinctrl: Use DEFINE_SHOW_STORE_ATTRIBUTE() helper for debugfs

Use DEFINE_SHOW_STORE_ATTRIBUTE() helper for read-write file to reduce some
duplicated code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Message-ID: <20240404193521.3581399-1-andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>