pinctrl: intel: mark intel_pin_to_gpio __maybe_unused

pinctrl: intel: mark intel_pin_to_gpio __maybe_unused

The intel_pin_to_gpio() function is only called by the
PM support functions and causes a warning when those are disabled:

drivers/pinctrl/intel/pinctrl-intel.c:841:12: error: unused function 'intel_pin_to_gpio' [-Werror,-Wunused-function]

Mark it __maybe_unused to suppress the warning.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Chris Chiu <chiu@endlessm.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>