[media] doc-rst: do a poor man's conversion of v4l2-framework

Make Sphinx happy with v4l2-framework.rst by putting all C
code inside code-block.

Please note that this is a poor man ReST conversion, as several
of those blocks should actually be converted to use :cpp:func:,
pointing to the kAPI auto-generated documentation.

The problem is that we currently lack kernel-doc documentation
for most of the stuff described there.

So, let's do a poor man's conversion. We should later address
this.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>