Documentation: devicetree: add vendor prefix for Pine64

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
[Maxime: Change title prefix to match the usual style]
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>