When we wrote the media controller's section, we re-used the
V4L, just because we were lazy to create a brand new DocBook.

Yet, it is a little ackward to have it mixed with V4L. So,
move it to its own directory, in order to have it better
organized.

No functional changes.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>