kernel: events: prevent k_event_init() from being called in an ISR

Most kernel objects should be initialized well before being
manipulated in ISR context.

Event objects are no exception. Initializing a k_event object in
ISR context would implicitly be racey and introduce an element of
non-determinism.

Assert that k_event_init() is not called from ISR context.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>