drivers: entropy: nrf5: Fix race condition on hardware access

Neither entropy_nrf5_get_entropy_isr() nor isr() protected
access to hardware. As result in some situations a random value
could be "stolen" by a higher priority call  interrupting one
of the mentioned functions.

This commit solves this problem by introducing random_byte_get()
which accesses hardware atomically.

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