This patch adds description for MT6779 IOMMU.

MT6779 has two iommus, they are mm_iommu and apu_iommu which
both use ARM Short-Descriptor translation format.

In addition, mm_iommu and apu_iommu are two independent HW instance
, we need to set them separately.

The MT6779 IOMMU hardware diagram is as below, it is only a brief
diagram about iommu, it don't focus on the part of smi_larb, so
I don't describe the smi_larb detailedly.

			     EMI
			      |
	   --------------------------------------
	   |					|
        MM_IOMMU                            APU_IOMMU
	   |					|
       SMI_COMMOM-----------		     APU_BUS
          |		   |			|
    SMI_LARB(0~11)         |	                |
	  |		   |			|
	  |		   |		   --------------
	  |		   |		   |	 |	|
   Multimedia engine	  CCU		  VPU   MDLA   EMDA

All the connections are hardware fixed, software can not adjust it.
Signed-off-by: Chao Hao <chao.hao@mediatek.com>
Reviewed-by: Rob Herring <robh+dt@kernel.org>
Link: https://lore.kernel.org/r/20200703044127.27438-2-chao.hao@mediatek.comSigned-off-by: Joerg Roedel <jroedel@suse.de>