drivers: entropy: nrf5: add "depends on MULTITHREADING"

entropy_nrf5 modules uses k_sem_xxx() functions in several
places, but these functions are only functional when
CONFIG_MULTITHREADING is set, otherwise they just fallback to
the empty weak implementation provided in
zephyr/lib/libc/armstdc/src/threading_weak.c.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>