media: platform: Makefile: reorganize its contents

There are lots of inconsistencies here: some directories are
included as-is, and others included using one (or more) symbols
that are inside it. Also, its entries are not sorted.

That makes it harder to maintain.

Reorganize it by placing everything on alphabetic order and
providing some hints about how patches for such file is expected.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>