arm64: mm: Fix a bug in iommu dma-mapping

iommu_alloc_attrs and arm_iommu_mmap_attrs calls
dma_pgprot with device type set to always 'coherent'
which is wrong. Should be based on the device type.

Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>