rtc: allow rtc_read_alarm without read_alarm callback

.read_alarm is not necessary to read the current alarm because it is
recorded in the aie_timer and so rtc_read_alarm() will never call
rtc_read_alarm_internal() which is the only function calling the callback.

Reported-by: Zhipeng Wang <zhipeng.wang_1@nxp.com>
Reported-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Fixes: 7ae41220 ("rtc: introduce features bitfield")
Tested-by: Philippe Schenker <philippe.schenker@toradex.com>
Link: https://lore.kernel.org/r/20230214222754.582582-1-alexandre.belloni@bootlin.com


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