PMU: Statically verify that STARTERP0 and STARTERP1 registers are configured correctly

[hannobraun]

To wake up from deep-sleep or power-down mode, the STARTERP0 and STARTERP1 registers need to be configured correctly. Otherwise an interrupt that is intended to wake the system up won't have the desired effect.

It would be nice if this error was prevented somehow, but I'm actually not sure how to achieve that. Maybe the methods to enter the respective low-power modes could have an additional argument to signal the user's intent about which interrupts should be able to wake the system up. Then this argument could be used to statically verify the STARTERP0/1 configuration.

But I don't know. All of this sounds like it could be part of some higher-level API, not necessarily the low-level methods to enter sleep mode. I'm open to suggestions!