ARM: sunxi: Fix GPLv2 wording

During the GPL to GPL/X11 licensing migration, the GPL notice introduced
mentionned the device trees as a library, which is not really accurate. It
began to spread by copy and paste. Fix all these library mentions to reflect
the file that it's actually just a file.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>