iommu: Provide dummy entries for DOMAIN_ATTR_ATOMIC

Map / unmap calls on legacy IOMMU driver are by
default atomic. But clients or test drivers might
still call set_attribute for targets not using
legacy IOMMU driver. So for sake of completeness
provide a dummy entry so that clients don't fail
on call set_attribute.

Change-Id: Ifc2567311ea4fe3cba109706d247ac653f06bbae
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>