drivers: clock_control_nrf: Fix releasing/stopping of HFCLK

Routines called by users to release (and perhaps stop) the HFCLK
cannot synchronize with only the `hfclk_users` atomic variable,
because a thread can be preempted right after it clears the proper
bit in that variable but before the HFCLK is actually requested to
stop, and another user can then request the HFCLK to start. This can
result in HFCLK being stopped right after it was requested to start
and in `hfclk_users` holding an incorrect value.
Fix this by locking interrupts in those routines until the HFCLK is
stopped.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>