dt-bindings: Document JZ47xx VPU auxiliary processor

Inside the Video Processing Unit (VPU) of the recent JZ47xx SoCs from
Ingenic is a second Xburst MIPS CPU very similar to the main core.
This document describes the devicetree bindings for this auxiliary
processor.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200515104340.10473-1-paul@crapouillou.net
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>