samples: nrf: system_off: show how to disable deep sleep

Due to a long standing difference of requirements enabling deep sleep
will by default cause any application that delays for an unbounded
period to power down.  On Nordic doing so turns the system off.  Show
how to prevent this from happening while still allowing deep sleep
states to be available for the application's controlled use.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>