The sample is currently to tied to nrf9160dk_nrf9160 board. Refact to
allow build with board and select between device power management on or
off. Move nrf9160dk_nrf9160.overlay to proper boards directory and add
nrf9160dk_nrf9160.conf to set the correct device. Device PM needs SYS
PM be implemented and is an specific configuration and for those boards
that have it implemented user can add overlay-pm.conf to enable the
feature.
Fixes#28094.
Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>
