rtc: snvs: Use __maybe_unused instead of #if CONFIG_PM_SLEEP

Use __maybe_unused for power management related functions
instead of #if CONFIG_PM_SLEEP to simply the code.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>