arm64: fix warnings without CONFIG_IOMMU_DMA

__swiotlb_get_sgtable_page and __swiotlb_mmap_pfn are not only misnamed
but also only used if CONFIG_IOMMU_DMA is set.  Just add a simple ifdef
for now, given that we plan to remove them entirely for the next merge
window.

Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Will Deacon <will.deacon@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>