Implement AON Timer driver for the RP2350

[Ragarnoy]

I recently needed an RTC, thought that the RP2350 didn't have one, but it actually does and has something that's kind of better (depending on your use case). This is my shot at implementing it

• Add Always-On (AON) Timer driver for RP2350 with async support
• Support configurable clock sources and alarm wake modes
• Include async example demonstrating timer usage

I did not implement the time driver on it because the resolution on it is so low that I doubt anyone would want to use it as their main timer. But I'm leaving it up here to decide, if people decide otherwise I can add the implementation. But imo this works better as a simple "global" clock that can be used to wake up from dormant.
I also was too lazy to have an example that wakes from dormant state

Also I think we can probably make the DateTime implementation in the rtc module common to both the AON and the rp2040's RTC, I didn't want to risk breaking stuff so I didn't do it but I think it's easily doable.