iommu: Document iommu_ops.is_attach_deferred()

Add missing kerneldoc for iommu_ops.is_attach_deferred().

Fixes: e01d1913 ("iommu: Add is_attach_deferred call-back to iommu-ops")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Joerg Roedel <jroedel@suse.de>