media: pixfmt-meta-d4xx.rst: Add a license to it

This file is a recent addition to media docs, and it is now the
only one without any license.

While the best is to dual-license with GFDL and GPL, it is,
at least, compatible with GFDL, as this is a requirement to be
part of the media uAPI docs.

So, add such license to it.

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