Add support for low-power modes

[maxgerhardt]

The core, or a library, should be able to expose the low-power functionality of the chip, such as

• a low-power run
• deep sleep
• standby (only wakeup through pin or reset, ..)

(I have not checked that these are the low-power modes available, just a guess from STM32 chips)

It should also be supported to put every known peripheral to sleep, or better yet, automatically put it to sleep if not needed.