media: docs: fix some GPL licensing ambiguity at the text

Those files are meant to be dual GPL 2.0 and GFDL without
implicit sections. However, by a wrong cut-and-paste, I ended
by applying a GPL 2+ license text to it, while still using the
GPL 2.0 SPDX tag, with would cause an ambiguity about the
licensing model.

Solve this by explicitly mentioning that the dual licensing
is between GPL 2.0 and GFDL and correcting the text.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>