dt-bindings: display: add Unisoc's dpu bindings

DPU (Display Processor Unit) is the Display Controller for the Unisoc SoCs
which transfers the image data from a video memory buffer to an internal
LCD interface.

Cc: Orson Zhai <orsonzhai@gmail.com>
Cc: Chunyan Zhang <zhang.lyra@gmail.com>
Signed-off-by: Kevin Tang <kevin.tang@unisoc.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211207142717.30296-4-kevin3.tang@gmail.com