[media] V4L2: add documentation for V4L2 clock helpers and asynchronous probing

Add documentation for the V4L2 clock and V4L2 asynchronous probing APIs
to v4l2-framework.txt.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>