media: dt-bindings: media: document allegro-dvt bindings

Add device-tree bindings for the Allegro DVT video IP core found on the
Xilinx ZynqMP EV family.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>