drivers: entropy: Port the LL rand driver to entropy

The rand driver present in the BLE Link Layer is able to provide
entropy for both threads and ISRs, and so it is more suited to be used
as a generic nRF5x entropy driver instead of the current one. This will
allow applications and the Link Layer to use the same driver without
duplicating it.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>