media: dvb uapi docs: better organize header files

Instead of having one chapter per file, place all of them at
the same chapter. That better organize the chapters at the uAPI
documentation.

As a side effect, now all uAPI headers are at the same page,
at the html output, with makes easier to use it as a reference
index for the spec.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>