iommu/arm-smmu: silently succeed the secure VMID domain attr

With the recent page table refactor we've lost our code to assign SMMU
page tables to secure VMIDs.  This functionality will be added back
later but for now just silently succeed when clients set this attribute.
This works for now because secure use cases haven't been brought up yet.

Change-Id: Ic2970e01f2899a9649c883f85aaaec828f2e7597
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>