RP2040: add Alarm module

[hierophect]

This PR adds the Alarm module for the raspberrypi port, with the TimeAlarm and PinAlarm alarm types (the RP2040 does not have a touch module). Tested on the Raspberry Pi Pico.

[hierophect]

One complication of this implementation was the RP2040's inability to wake off the RTC when in "dormant" mode, unless the RTC is supplied with an external clock input from a GPIO pin while sleeping. There are a couple options for how to approach this, and I'm not sure which is the best fit for Circuitpython:

1. Throw a ValueError when attempting to use a TimeAlarm with deep sleep (current implementation)
2. Allow RTC Alarms, but trust the user to supply a clock source. This isn't very Circuitpython-y, but could maybe be augmented by extra checks or something?
3. Defer to the lower tier of sleep when a TimeAlarm is included in the alarm list, but gate the clocks very severely. This avoids any user-facing changes but can only obtain a minimum power draw of ~5mA, so it isn't very good deep sleep performance. It might seem like a mysterious degradation in battery life from the user's perspective when compared to only using PinAlarms.

[dhalbert]

3. It might seem like a mysterious degradation in battery life from the user's perspective when compared to only using PinAlarms.

On other boards the opposite is true (though not 5mA, but 1.5mA, say): PinAlarms are more expensive in terms of power than TimeAlarms. So I'm not sure this is terrible, though it is disappointing.

[dhalbert]

Hi - some build issues due to board_deinit() not found.

[hierophect]

@dhalbert think you could take another look at this this week?

[dhalbert]

Sorry, I missed this was ready for re-review.

[dhalbert]

Thanks for your patience. Let's go with this for now, and we can think about improvements later.