media: v4l2-event: keep track of the timestamp in ns

Internally use ktime_get_ns() to get the timestamp of the event.
Only convert to timespec when interfacing with userspace.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>