random: rand32_entropy_device: get entropy device using DEVICE_DT_GET

THe entropy device can be ontained at compile time. Simplify some
logs/code as a result. Also adjusted "entropy_driver" name to
"entropy_dev", since it holds a "device", not a "driver".

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>