media: docs: brainless mass add SPDX headers to all media files

All Documentation files outside the uAPI are all licensed with,
at least, GPL 2.0. So, mark them as such.

The ondes at media/uapi are at least GFDL 1.1+.

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