samples: boards: nrf52: test demo for nrf52 power management

This sample tesecase provides sample code :-
1. Triggers Low Power tasks into nrf52 SOC (CONST LAT and LOW PWR)
2. Triggers System Off state.
3. Wake up device through port Event by GPIO (Button 1).

Signed-off-by: Youvedeep Singh <youvedeep.singh@intel.com>