media: ov5670: Add implementation for events

Use v4l2 control API helpers to support the events.

Fixes v4l2-compliance:

test VIDIOC_(UN)SUBSCRIBE_EVENT/DQEVENT: FAIL

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>