rtc: ds1511: implement ds1511_rtc_read_alarm properly

ds1511_rtc_read_alarm was useless as it is only called at boot time so
the alarm members of pdata have not yet been set. Read the actual registers
so there is a chance to get a meaningful value.

Then, drop the alarm related members of pdata as they are not used anymore.

Link: https://lore.kernel.org/r/20240227230431.1837717-10-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>