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>

arch/arm64/mm/dma-mapping.c

@@ -160,6 +160,7 @@ void arch_sync_dma_for_cpu(struct device *dev, phys_addr_t paddr,
 	__dma_unmap_area(phys_to_virt(paddr), size, dir);
 }
 
+#ifdef CONFIG_IOMMU_DMA
 static int __swiotlb_get_sgtable_page(struct sg_table *sgt,
 				      struct page *page, size_t size)
 {
@@ -188,6 +189,7 @@ static int __swiotlb_mmap_pfn(struct vm_area_struct *vma,
 
 	return ret;
 }
+#endif /* CONFIG_IOMMU_DMA */
 
 static int __init atomic_pool_init(void)
 {