treewide: rename EVENT_CLOCK to CLOCK

treewide: rename EVENT_CLOCK to CLOCK

While we use it for edge event timestamps exclusively at the moment,
the actual enum names shouldn't limit its application and simply just
refer to existing clock types known by the GPIO uAPI. The relevant
functions are still called set/get_event_clock() as it's in line with
their functionality.

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