rtc: xgene: mark PM functions as __maybe_unused

The new xgene_rtc_alarm_irq_enabled() function is only accessed
from PM code, which is inside of an #ifdef; this causes a harmless
build warning when CONFIG_PM is disabled:

drivers/rtc/rtc-xgene.c:108:12: error: 'xgene_rtc_alarm_irq_enabled' defined but not used [-Werror=unused-function]

Just remove the #ifdef and use __maybe_unused annotations instead,
to make the code more robust here.

Fixes: d0bcd82b

 ("rtc: xgene: Fix suspend/resume")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Loc Ho <lho@apm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>