rtc: sc27xx: Always read normal alarm when registering RTC device

When registering one RTC device, it will check to see if there is an
alarm already set in RTC hardware by reading RTC alarm, at this time
we should always read the normal alarm put in always-on region by
checking the rtc->registered flag.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>