WIP: Add alarms for RTC

[chrenderle]

Didn't compile it yet.
Didn't test it on any target yet.
Only written for STM32L0 and probably breaks other targets.

TODO:

• make it compile ready
• test on STM32L0
• test compilation on other targets
• write drivers which allow setting the RTC alarm

[xoviat]

For the STM32L0 the interrupt routine is the same as the wkup timer so there has to be some overlap.

If that is the case, the low-power wakeup irq handler needs to be converted to bind_interrupts and moved to the rtc mod, and needs to call the on_wakeup_irq function when the alarm wakes the chip.

The code in low power and time driver is only there since for the low-power feature the low-power executor has ownership of the rtc.

The low-power feature is only intended to have ownership over part of the rtc: it's wakeup functionality and timekeeping. The alarm functionality can be split off like this:

let rtc = Rtc::new(...)
let alarm1 = rtc.alarm1.take().unwrap();

let rtc=make_static!(rtc);
stop_with_rtc(rtc);

alarm1.wait_for_alarm(day, hour, minute, second).await;

In fact, low-power should be able to work without embassy-time at all (in that case, it would not even set the wakeup alarm) and alarms should be available to work without the low-power feature (in that case, the core wouldn't stop, but sleep until the alarm).