Enable IDLE and Deep Powerdown mode.
IDLE is pretty straightforward - insteady of busy waiting, the CPU will
enter an idle state from which it will resume on any event.
Deep Power Down shuts off the entite system except for the battery backup
power domain.
That means the CPU will reset on resume and can be woken by e.g. RTC.
SLEEP and POWERDOWN disable the PLL and the PLL and Flash respectively.
This requires some proper wake-up handling.
Since this turned out to be a major time sink and those modes are never
currently never used in RIOT outside of tests, I left this as an exercise
for a future reader.
