Since the "EXTI->PR" is an "rc_w1" type of register, we need to be
careful when clearing our interrupt flag in the register. When there
are multiple interrupt flags set in the register, the "|=" operation
will mistakenly clear all pending interrupts instead of just ours.
A naive implementation may set a RTC alarm in 30s by calling
struct tm now;
rtc_get_time(&now);
now.tm_sec += 30;
rtc_set_alarm(&now, _cb, NULL);
This works for RTC implementations that use a RTT internally and call
mktime() to convert the struct tm to a unix timestamp, as mktime() will
normalize the struct in the process.
Call rtc_tm_normalize() when the RTC uses separate registers for time / date
components to ensure it is normalized.
This also modifies tests/periph_rtc to exercise this case.
