media: vimc.rst: add vimc-streamer source documentation

Since vimc-streamer.{c, h} are fully documented and conforming with the
kernel-doc syntax, add those files to vimc.rst

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Suggested-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>