media: dt-bindings: allwinner: document H6 Hantro G2 binding

Allwinner H6 contains older Hantro G2 core, primarly used for VP9 video
decoding. It's unclear for now if HEVC is also supported.

Describe its binding.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
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>