pinctrl: samsung: mark PM functions as __maybe_unused

The rework of the suspend/resume handling uses the wrong #ifdef check, leading
to a build warning without CONFIG_PM_SLEEP:

drivers/pinctrl/samsung/pinctrl-samsung.c:1142:12: error: 'samsung_pinctrl_resume' defined but not used [-Werror=unused-function]
drivers/pinctrl/samsung/pinctrl-samsung.c:1092:12: error: 'samsung_pinctrl_suspend' defined but not used [-Werror=unused-function]

Using a __maybe_unused annotation instead of the #ifdef is a simple way to
avoid this problem class.

Fixes: 2b24efa8

 ("pinctrl: samsung: Replace syscore ops with standard platform device pm_ops")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>