kernel: use DEVICE_DT_GET_OR_NULL for entropy device
kernel: use DEVICE_DT_GET_OR_NULL for entropy device

A reference to the entropy device can be obtained at compile time, so
avoid using device_get_binding().

Signed-off-by: Gerard Marull-Paretas <bedb45f43a286e49f4dcda039a436b216ae61ef3@nordicsemi.no>

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