media: vimc: documentation for lens

Add documentation for vimc-lens.
Add a lens into the vimc topology graph.

Signed-off-by: Yunke Cao <yunkec@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>