iommu: Mark __iommu_map_sg() as static

Now __iommu_map_sg() is used only in iommu.c file, so mark it static.

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/ab722e9970739929738066777b8ee7930e32abd5.1591930156.git.baolin.wang@linux.alibaba.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>