Shows an example of how to sleep from LP3 (run mode) and wake-up
from LP2, LP1 and LP0 by using the GPIO wake-up triggers

Usage
1. On power up, device is in LP3 (LED3 on)
2. Push SW3 push button and the device will go to LP2 sleep mode (LED2 on)
3. Push SW3 again to wake up from LP2 and enter LP3 (LED3 on)
4. Push SW2 push button and the device will go to LP1 sleep mode (LED1 on)
5. Push SW2 again to wake up from LP1 and enter LP3 (LED3 on)
6. Push SW1 push button and the device will go to LP0 sleep mode (LED0 on)
7. Push SW1 again to wake up and restart program (three blinks on LED3 then stays on)