ARM: at91: fix GPLv2 wording

During the submission of these new sama5d4 files, the GPL notice mentioned the
device tree as a library, which is not really accurate.
Fix all these library mentions to reflect the fact that it's actually just a
file.

Reported-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>