dt-bindings: iommu: add bindings for sprd IOMMU

This IOMMU module can be used by Unisoc's multimedia devices, such as
display, Image codec(jpeg) and a few signal processors, including
VSP(video), GSP(graphic), ISP(image), and CPP(camera pixel processor), etc.

Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Link: https://lore.kernel.org/r/20210305093216.201897-2-zhang.lyra@gmail.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>