rtc: ds1511: set alarm offset limit

The ds1511 can only support alarms up to a month in the future (which we
currently limit to 28 days).

Link: https://lore.kernel.org/r/20240227231356.1840523-2-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>