MAINTAINERS: update documentation for media subsystem

With ReST conversion, the media subsystem documentation is now
located on different directories.

Update them.

Suggested-by: Joe Perches <joe@perches.com>
Cc: LKML <linux-kernel@vger.kernel.org>
Cc: linux-doc <linux-doc@vger.kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>