[media] doc-rst: add CEC header file to the documentation

Adding the header file is interesting for several reasons:

1) It makes MC documentation consistend with other parts;
2) The header file can be used as a quick index to all API
   elements;
3) The cross-reference check helps to identify symbols that
   aren't documented.

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