devicetree: add vendor prefix for Vivante Corporation

Trivial patch to add Vivante Corporation to the list of
devicetree vendor prefixes.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>