media: dt-bindings: media: add document for STM32 DMA2d bindings

This adds documentation of device tree bindings for the STM32 DMA2D

Signed-off-by: Dillon Min <dillon.minfei@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>