rtc: s5m: signal the core when alarm are not available

Clear the RTC_FEATURE_ALARM bit to signal to the core when alarms are not
available to ensure the alarm callbacks are never called and userspace is
aware alarms are not supported.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210804104133.5158-2-alexandre.belloni@bootlin.com