treewide: use plural 'events' in read_edge_event() functions

treewide: use plural 'events' in read_edge_event() functions

The read_edge_event() family of functions should actually be called
read_edge_events() as they universally allow to read more than one
event. We're converting wait_edge_event() too for consistency.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>