iommu: Document iommu_ops.iotlb_sync_map()

Add missing kerneldoc for iommu_ops.iotlb_sync_map().

Fixes: 1d7ae53b ("iommu: Introduce iotlb_sync_map callback")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>