ath79: relicense DTS files to the GPL 2.0+ / MIT

ath79: relicense DTS files to the GPL 2.0+ / MIT

Some maintainers prefer DTS files licensed under permissive license like
MIT / BSD. As all DT bindings should be OS independent and DTS files are
pretty separated from Linux code it probably makes sense to share them
across projects.

The safest solution is to use dual licensing: that way it stays clear
these files can be used in GPL projects without depending on current
belief of licenses compatibility.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: John Crispin <john@phrozen.org>