media: i2c: st-vgxy61: Add V4L2_SUBDEV_FL_HAS_EVENTS and subscribe hooks

Any V4L2 subdevice that implements controls and declares
V4L2_SUBDEV_FL_HAS_DEVNODE should also declare V4L2_SUBDEV_FL_HAS_EVENTS
and implement subscribe_event and unsubscribe_event hooks.

With that done, v4l2-compliance testing is now ok.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>