media: dt-bindings: media: Add Allwinner A31 ISP bindings documentation

This introduces YAML bindings documentation for the Allwinner A31 Image
Signal Processor (ISP).

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>