media: dt-bindings: Document i.MX8MQ VPU bindings

Add devicetree binding documentation for the Hantro G1/G2 VPU on i.MX8MQ.

Signed-off-by: Philipp Zabel <p.zabel@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+huawei@kernel.org>