media: dt-bindings: media: Add Allwinner A31 MIPI CSI-2 bindings documentation

This introduces YAML bindings documentation for the Allwinner A31 MIPI
CSI-2 controller.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>