random: rand32_ctr_drbg: obtain entropy device using DEVICE_DT_GET

The entropy device can be obtained at compile time. Adjusted the
assertion message slightly, as it no longer uses the entropy macro
label. Also adjusted the entropy_driver variable to entropy_dev, since
it does not hold a "driver" but a "device".

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