If you call rtc_get_datetime immediately after rtc_set_datetime you get
junk back. According to the datasheet "Writing to the RTC will take
2 clk_rtc clock periods to arrive". So add a delay after calling
rtc_set_datetime in aon_timer_set_time_calendar.
Fixes#2148
* ta_set_timeout can fail to set an alarm
If alarm_pool_irq_handler takes <1us between handling an alarm and
calling ta_set_timeout then no alarms will be set as it will appear as
if an earlier alarm is already armedi before the target time.
Make sure ta_set_timeout always leaves with an alarm set by checking the
armed status.
Fixes#2118
* ta_disable_irq_handler should unarm its timer
