arm64: dma-mapping: use correct type for iova

IOMMU virtual addresses use the dma_addr_t type since they can be up to
64-bits.  We're currently using an `unsigned int' to store our IOVA in
arm_iommu_map_sg, which could result in truncation.  Use the correct
type for an I/O virtual address: dma_addr_t.

Change-Id: Ie63bf17268ca70d102ab9d472ed9bcc6f4a793d7
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>