ARM: dts: msm: use 'dma-coherent-hint-cached' for fastrpc nodes

Use 'dma-coherent-hint-cached' instead of 'dma-coherent' for the
fastrpc iommu context banks.

Now, on a QGKI kernel, cached ION buffers will be DMA mapped as
IOMMU cached for the fastrpc devices, all other buffers will be
DMA mapped as IOMMU uncached and dma_alloc_attrs will always
return memory with a cached CPU mapping as well as DMA mapping
its memory as IOMMU cached.

On a GKI kernel, all buffers for fastrpc devices will be DMA
mapped as IOMMU uncached and dma_alloc_attrs will always return
CPU uncached memory.

Change-Id: I4c5b3e082dc057cb76bfd039b19c6211c729597f
Acked-by: Thyagarajan Venkatanarayanan <venkatan@qti.qualcomm.com>