arm64: dma-mapping: fix build when !CONFIG_ARM64_DMA_USE_IOMMU

When CONFIG_ARM64_DMA_USE_IOMMU is not selected, drivers that make use
of the ARM DMA IOMMU mapping APIs currently don't link.  First instinct
might be to add a dependency on CONFIG_ARM64_DMA_USE_IOMMU to those
drivers, but they might not actually want to do that because they might
have other ways of getting DMA-able memory.

Allow compilation when CONFIG_ARM64_DMA_USE_IOMMU is not selected by
providing necessary stub functions.

Change-Id: I172e00a0748c70676b8ff7555e217a1e6122e3e6
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>