rtc: pcf8563: let the core handle the alarm resolution

Set RTC_FEATURE_ALARM_RES_MINUTE, so the core knows alarms have a
resolution of a minute. Also, the core will properly round down the alarm
instead of up.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220309162301.61679-16-alexandre.belloni@bootlin.com