rtc: s5m: enable wakeup only when available

Call device_init_wakeup() only when alarms are available and the RTC is
actually able to wake up the system.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210804104133.5158-3-alexandre.belloni@bootlin.com