drivers: entropy: nrf5: Replace random number pool implementation

The old algorithm managing random number pools was inefficient and
prone to race conditions. Moreover, its behaviour was not compatible
with the API (provided buffers were filled from the end, leaving
beggining of the buffer untouched if there was not enough random data).

The new implementation is a bit faster and it is race-free as long as
all callers of the entropy_get_entropy_isr() are executed in
"run-to-completion" mode.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>