Added (deep)sleep to KL25

Sleep is regular sleep (wait mode). Deepsleep sets it to very-low power
stop mode (fairly similar to what NXP calls power-down).

Two notes:
After uploading new code the KL25 needs its power removed and enabled
again, otherwise it stays in a kind of debug mode from programming,
resulting in increased power consumption.

The interrupt waking up the device will run directly on the crystal,
once that interrupt is serviced it will turn back on the PLL (if it used
to be enabled)