samples: nrf: system_off: demonstrate use of device pm to reduce power

Currently system power management does not automatically control
device power, and this example demonstrates high current draw due to
the UART being enabled.  Enable device power management and
demonstrate how to manually turn off greedy devices.

Also remove the 60 s delay that confirms we successfully turned off
the residency policy; that's not directly relevant to the purpose of
the sample.

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