lib: os: poweroff: Disable ZLIs before poweroff

We currently only disable "normal" IRQs with irq_lock(). This is not
sufficient if ZLIs are enabled, as even though they are supposed to
be "above" the kernel, they must not interrupt the poweroff procedure.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>