pinctrl: msm: mark PM functions as __maybe_unused

Without CONFIG_PM_SLEEP, we get annoying warnings about unused functions:

drivers/pinctrl/qcom/pinctrl-msm.c:1082:12: error: 'msm_pinctrl_resume' defined but not used [-Werror=unused-function]
 static int msm_pinctrl_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~~~
drivers/pinctrl/qcom/pinctrl-msm.c:1075:12: error: 'msm_pinctrl_suspend' defined but not used [-Werror=unused-function]
 static int msm_pinctrl_suspend(struct device *dev)

Mark them as __maybe_unused to shut up the warning and silently drop
the functions without having to add ugly #ifdefs.

Fixes: 977d057a ("pinctrl: msm: Add sleep pinctrl state transitions")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Evan Green <evgreen@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>